The Galaxies of Autumn

As the Earth moves in its orbit around the sun, new constellations are revealed in the east as the old ones disappear into twilight. The bright stars of the Summer Triangle will soon be replaced by the even brighter stars of Orion and his winter companions.

Use this chart to locate the galaxies of autumn. Credit: Starry Night Software

During this period of transition, we find ourselves looking outwards from our own galaxy into intergalactic space. This is one of the two best times of the year to hunt for galaxies.

The stars of autumn are relatively faint, as we are looking out away from the disk of our galaxy. To see these fainter stars you may need to travel away from your urban or suburban home to seek out the darker skies of the countryside.

After twilight falls, look eastward to see the Square of Pegasus, formed by four second magnitude stars, about the same brightness as the stars of the Big Dipper. As the Square is rising in the east, it will appear more as a diamond than a square. The leftmost star of the diamond, Alpheratz, marks the head of Andromeda. Two streams of stars extend to the left from Alpheratz, pointing towards Perseus, just below the "W" shape of Cassiopeia.

These constellations provide the framework of relatively nearby stars in our own galaxy, the Milky Way, through which we look locate our neighboring galaxies beyond.

Most people have never seen another galaxy. In fact, most people today have never seen the galaxy in which we live, because the widespread glow of light pollution blocks the Milky Way's faint light. It’s only on rare occasions of major power failures that city dwellers get to see the Milky Way.

To locate our nearest galaxies, look for the middle star in the lower chain of Andromeda, Mirach. Just above it is a fainter star in the upper chain. Draw an imaginary line from Mirach to this second star, and extend it the same distance. This will take you to the Andromeda Galaxy.

What will you see? With the unaided eye, probably nothing, unless you are at a very dark location. However, with ordinary 7x50 or 10x50 binoculars, you will see a tiny faint glowing cloud. Oddly enough, if you point a telescope at this cloud, you will probably see less than with binoculars. This is a case of less being more: the wide field of view of the binoculars sets off the view of the Galaxy perfectly.

This faint glowing cloud may not seem impressive, until you realize that its light has been traveling for more than two million years to reach your eyes. When that light began its journey, our ancestors were just a bunch of small primates wandering on the plains of Africa.

Go back to Mirach and its companion, but this time, follow the line in the opposite direction. This will lead you to an even fainter cloud, the Triangulum or Pinwheel Galaxy. This is a smaller galaxy than Andromeda, but located at about the same distance from us. Even though this is one of the brightest and nearest galaxies in our sky, it is unusually difficult to see. That’s because it is almost at right angles to our line of vision, so we are seeing it in plan view. As a result its feathery edges blend into the background, and we have no sharp edge to catch our eye.

Usually you need binoculars to see the Triangulum Galaxy, but sharp eyed observers at very dark locations have managed to see it with the naked eye, making it the farthest object that the human eye can see unaided.

Partial Eclipse of the Moon

There will be a partial eclipse of the moon on Friday October 18. The moon will pass through the edge of the Earth’s shadow, ending up partially in the shadow of the Earth.

The moon passes through the edge of the Earth’s shadow at dusk on Friday October 18, as seen from eastern North America.  Credit: Starry Night Software

If you look closely at any shadow cast by the sun, say the one cast by your hand on a piece of paper, you will notice that the edge of the shadow isn’t sharp. That’s because the sun is not a point source of light. It is a disk, so the shadows it casts are slightly fuzzy. The solid dark part of the shadow is called the “umbra,” Latin for “shade.” The fuzziness is called the “penumbra,” Latin for “almost shade.”

When a shadow is cast by a nearby object, the penumbra is very slight. When the shadow is as far away as the moon is from the Earth, the penumbra is quite wide.

The graphic shows the situation at maximum eclipse at 7:50 p.m. EDT on Friday night, October 18. The inner circle is the umbra, the outer circle the penumbra. The moon makes it part way into the penumbra, but misses the umbra completely, hence this is a “partial penumbral eclipse.”

For observers in Africa, Europe, and western Asia, the eclipse will occur in the middle of the night when the moon is high overhead. The partial shading will be visible as a slight reddish dimming of the normally bright full moon.

For observers in North America and South America, maximum eclipse will be around the time of moonrise, which is also the time of sunset. This will make the eclipse difficult to see, because we will be looking for a slight dimming of a moon which is already dimmed by passing through a great deal of the Earth’s atmosphere.

The farther east and north you are located, the better your chances of seeing this eclipse. For example, in New York City, the moon will rise at 5:59 p.m. EDT, and will be at an altitude of 20 degrees at maximum eclipse. In Chicago, moonrise is at 5:54 p.m. CDT and the moon’s altitude only 9 degrees at maximum eclipse. In Los Angeles, moonrise is at 6:09 p.m. PDT, more than an hour after maximum eclipse, so the chances of seeing the eclipse are zero.

For observers in North America, the effects of the moon’s shadow will be most pronounced on the lower right corner of the moon. The shadow will probably be more pronounced in photographs than with the naked eye, so this is a good opportunity to get out your telephoto lens and photograph the rising moon. Remember that maximum eclipse will be at 7:50 p.m. EDT and 6:50 p.m. CDT.

Triple Eclipses on Jupiter

A rare planetary viewing event occurs this week when three of Jupiter’s largest moons cast their shadows simultaneously on the planet below them: three solar eclipses at the same time.

Three of Jupiter’s moons cast their shadows simultaneously on the planet beneath them on Friday night and Saturday morning, October 11 and 12. Picture the sun coming over your left shoulder, causing the three moons on the left to cast their shadows on the planet to the right, causing eclipses of the sun in three different locations on Jupiter. Credit: Starry Night Software

An eclipse of the sun happens when a moon casts its shadow on the planet below it. For observers located where the shadow falls, the sun appears to be completely blocked by the moon, and they are able to see the prominences and corona around the occluding moon.

In the Earth-moon system, the Earth is a fairly small target and its moon is far away, so it’s rare for the moon’s shadow to fall on the Earth. On average it happens about twice a year, and the diameter of the shadow on the Earth’s surface is quite small, only a couple of hundred miles in diameter. To see a solar eclipse, you need to be in exactly the right place, the place where the moon’s shadow falls.

Because our moon’s orbit is tilted with respect to the Earth, most of the time the moon’s shadow passes above or below the Earth, and no eclipse occurs. Only about twice a year do the three bodies line up exactly so that the moon’s shadow touches Earth. This next happens on November 3.

With Jupiter, the situation is different. Jupiter has four large moons: Io, Europa, Ganymede, and Callisto. These are relatively close to Jupiter and Jupiter is much larger than Earth. As a result, the shadows of Jupiter’s moons cross its face very frequently. The innermost moon Io causes an eclipse on Jupiter once every 1.8 days (42 hours). Even the outermost moon, Callisto, traveling much more slowly, should cause an eclipse every 17 days. In fact it does so less frequently because, like our moon, sometimes its shadow passes above or below Jupiter.

So, if eclipses on Jupiter happen very often, why don’t we see more of them? The timing has to be exactly right. Io may cause an eclipse on Jupiter every 42 hours, but the eclipse itself lasts only a little over 2 hours. Also, Io’s shadow is very small; you need a telescope with at least 90mm aperture to see it. If you aren’t looking for it, you probably wouldn’t see it at all, it’s that small. Also, because of the Earth’s rotation, Jupiter is below the horizon half the time, and often lost in the daylight sky.

If the shadows of its moons fall so often on Jupiter, what are the chances of two shadows falling simultaneously? Pretty good, it turns out. This is especially true because there is a resonance between the orbits of Jupiter’s satellites. Europa’s period of revolution is almost exactly twice that of Io, and Ganymede’s almost exactly four times. Only Callisto doesn’t keep step with the inner satellites.

As a result, double shadow transits usually happen in a group. The current group started with a double transit of Io and Europa’s shadows on September 28 and will continue every few days until November 13.

The rarest of all shadow transit events is when three shadows cross Jupiter’s face simultaneously, and this will happen this coming Friday night, October 11, stretching on into Saturday morning, October 12. Because of Jupiter’s present location, this event is mainly visible in the eastern part of North America. The event is in progress when Jupiter rises around midnight on the east coast, but is completely over by the time Jupiter rises on the west coast. Here are the times of the events to look for in Eastern Daylight Time—some events occur before Jupiter rises:

11:12 p.m. Callisto’s shadow enters

11:24 p.m. Europa’s shadow enters

12:32 a.m. Io’s shadow enters: all three shadows visible

1:37 a.m. Callisto’s shadow leaves

2:01 a.m. Europa’s shadow leaves

2:44 a.m. Io’s shadow leaves

Keep an eye on the moons themselves, because they will also begin to cross Jupiter’s disk: Io at 1:48 a.m. and Europa at 2:02 a.m. There’s an added bonus in the Great Red Spot also transiting at this time. It helps to visualize the moons’ movements in three dimensions, with the sun seeming to come from over your left shoulder. Sometimes you get an almost three-dimensional effect with the moons casting their shadows on the planet beneath.

If you miss this event, there will be three double shadow transits later this month visible over most of North America, on October 16/17, 18/19, and 25/26. In each case, the events mostly occur after Jupiter rises around midnight on the first date, so that’s the night you should mark on your calendar. Remember that the date changes at midnight.

Starhopping to Uranus

Most of the planets in the solar system hover close to the sun, which illuminates them and makes them among the brightest objects in the sky. The outer planets, Uranus and Neptune, are far from the sun and catch few of its rays, making them dim and hard to find.

The planet Uranus is currently located in the dim constellation of Pisces. Although just visible with the unaided eye, Uranus is brighter than any of the stars in this area, so is relatively easy to locate with binoculars. Credit: Starry Night Software

Uranus, for example, reaches opposition with the sun on Thursday October 3. Directly opposite the sun in our sky, it is at its closest and brightest, yet it is only just barely visible to the unaided eye in a dark moonless sky. For most people, a binocular will be an essential tool for spotting Uranus.

This is done through a technique popular with amateur astronomers called “starhopping.” We use the brighter stars as guideposts to locating faint or distant objects. In this case, we start with the most prominent group of stars in the autumn sky: the four stars that form the Square of Pegasus. Although not among the brightest stars, these are all good second magnitude stars, about as bright as the stars in the Big Dipper.

Many people who go looking for the Square of Pegasus miss it because it is so large. The four sides of the Square are roughly 15 degrees long, about the length of the handle of the Big Dipper. Look for them in the east as the sky gets dark around 9 p.m. At that time the Square is rising, so is tilted over, making it more of a diamond shape than a square.

Now we begin our “starhop.” We start with the two stars that form the bottom of the square (if you’re in the northern hemisphere). Make that one side of a south-pointing equilateral triangle. The southern point of the triangle marks the “Circlet,” part of the dim constellation of Pisces. This is an oval of dim stars, more easily seen in binoculars than with the unaided eye.

To the left of the Circlet is a chain of stars, part of the chain that binds the two fish of Pisces together. The first two stars in this chain are a bit brighter than the rest. About half way between these two stars, and a bit to the south, you should find Uranus.

How will you know you have found it? First of all, although dim, Uranus is brighter than any of the stars in this area. Secondly, it has a distinctive blue-green color, quite unlike any star. Finally, if you plot its position relative to the stars, and check again in a few days, you will find that it has moved to the right.

Of course, the acid test is to point a telescope at it. Most newcomers to astronomy are surprised at how small any of the planets appear in a telescope, but Uranus is smaller still, a mere 3.7 arc seconds in diameter, about a tenth the size of Jupiter. Unless you have a very large telescope, it will appear as nothing more than a tiny blue-green pinhead.

Comet ISON's flyby of Mars

Today, comet ISON will fly by Mars at a distance of 0.07 AU.  The satellites in orbit around Mars and the rovers on the ground are at the ready to snap their pictures.  Thankfully, Curiosity has been designated "essential personnel" and will be operational despite the US government shutdown.

View of ISON from Mars (rendered by Starry Night)

Autumn Monsters

As the constellations of summer depart from our sky, they are replaced by what are often called "the watery constellations." These include normal sea creatures like fishes and dolphins, and even Aquarius carrying a water jug.

The constellations of autumn include some strange monsters like Capricornus, the Sea Goat, Cetus, the Sea Monster, and Pegasus, the Flying Horse. Credit: Starry Night Software

Among these watery creatures are some strange creatures which we would call monsters: strange combinations of parts of unrelated animals.

The first to appear is Capricornus, the Sea Goat. Seen in the lower right of our chart looking southward on an autumn evening in the northern hemisphere, he combines the front end of a goat with the rear end of a fish. Most people would be hard pressed to see either a goat or a fish in this large triangular group of stars. I see it more as a tricorn hat turned upside down. The front end of the goat, to the right, is marked by two wide double stars, Algedi and Dabih, a fine sight in binoculars. “Algedi” or “Al Giedi” is Arabic for “the goat.” The rear end of the fish is marked by Deneb Algiedi, which translates from Arabic as "the tail of the goat."

Much of our knowledge of ancient astronomy, along with mathematics and other sciences, has been passed down to us by medieval Arab scholars. In the process many of the old star names were translated into Arabic. As a result, astronomers learn a bit of Arabic. "Deneb" is Arabic for tail, so turns up in many star names in constellations derived from animals. The most famous is Deneb in Cygnus, marking the tail of the Swan.

"Al" is Arabic for "the" and turns up in many scientific words like "algebra," "alcohol," and "alkali."

In the lower left corner of our chart we find another monster, Cetus. Modern astronomy books usually translate this as "the whale," but our chart shows a much stranger creature. it has the head of a dragon, webbed feet, and a fishy tail. This tail is marked by one of the few bright stars in this part of the sky, Deneb Kaitos. With our new knowledge of Arabic, we can translate this easily as "the tail of the whale."

Buried in the heart of Cetus is a remarkable star called Mira, which means “wonderful” in Latin. This was discovered by David Fabricius in 1596 to be a star which varies in brightness, one of the first variable stars to be discovered.

Flying high above these watery creatures is yet another monster, a horse with wings called Pegasus. This is probably one of the most familiar mythological creatures, so familiar that most people never think of how strange a flying horse would be. The celestial flying horse is marked by four fairly bright stars forming an almost perfect square, the Square of Pegasus.

When I first went looking for Pegasus in the sky, I made a common beginner's error. Because I was using a small star chart, I looked for a small square of stars in the sky, and totally missed it. The constellations in the sky are much larger than they appear on star charts. So look for a really large square of stars.

Actually, only three of the four stars in the Square are part of Pegasus. The star in the upper left corner is Alpheratz, actually part of the constellation of Andromeda. But that is another story.

The September Equinox

Most people know that something called the “equinox” happens twice every year around March 21 and September 21, but don’t really know what that means. Here are the real facts about the equinox.

The sun crosses the celestial equator on September 22 at 4:44 p.m. EDT. On this date the sun rises due east and sets due west, and the day and night are of equal length. Image Credit: Starry Night Software

The Earth moves in two different ways. First, it spins on its polar axis, a line through the north and south poles, once every 24 hours, causing the alternation of day and night. Secondly, it moves in its orbit around the sun once every 365 1/4 days, causing the annual cycle of seasons. The equinox occurs when these two motions intersect.

Because the Earth is very massive, its mass has an enormously powerful gyroscopic effect. For this reason, its poles always point in the same direction, although a major earthquake can cause tiny wobbles in this axis. Most importantly, the Earth’s motion around the sun has absolutely no effect on the direction the poles are pointing, which has very important consequences for Earth’s seasons.

Astronomers mark the positions of objects in the sky relative to the Earth’s poles of rotation: those are the red lines you see in the chart. The most important line is the celestial equator, which divides the sky into northern and southern hemispheres.

The Earth’s pole of rotation is tilted 23.4 degrees relative to the plane of its orbit. This tilt is always towards the same point in the sky, called the celestial pole, no matter where in its orbit around the sun the Earth happens to be. This tilt has the effect on the surface of the Earth that the sun appears to move across the sky at an angle to the celestial equator. This is marked by the green line in the chart, called the “ecliptic” because eclipses happen along this line.

Twice a year, the sun crosses the celestial equator, moving from the northern hemisphere to the southern hemisphere or from the southern hemisphere to the northern hemisphere. These two crossings are very important for the inhabitants of Earth, because they mark the change in the direction the sun’s rays fall on the Earth.

Specifically, on September 22, the sun will move from the northern hemisphere to the southern hemisphere. It will pass overhead everywhere along the Earth’s equator on that date. It will rise exactly in the east and set exactly in the west. After that date, the sun will shine more on the southern half of our planet, and less on the northern half. Summer will be over in the northern hemisphere, and winter will be on its way. Also, winter will be over in the south, and summer will begin.

The sun will continue on its path southward for the next three months, reaching its southernmost point on December 21, the date called “solstice.” In the northern hemisphere, the days will become shorter, the nights longer, and the temperatures colder, all the result of the sun’s being south of the celestial equator.

It’s always important to remember that this is part of a cycle, and that after December 21 the sun will start moving northward again, and spring will be on its way.

First Quarter Moon

The next few nights are the best this month to observe the surface features of the moon. The sun will be rising along the center line of the moon, casting the mountains and craters in high relief.

Image Credit: Starry Night Software

The sun will be rising along the center line of the moon, casting the mountains and craters in high relief.

Beginners in astronomy are often surprised that the best time to study the moon is not at full moon, which will occur on Thursday the 19th this month. At full moon the sun is high overhead in the center of the moon, and the surface looks like the desert at high noon. The best time is when the sun is falling obliquely, casting long shadow, which is what you see at the first and third quarters.

Third quarter is a bit of a problem because it occurs when the moon is in the dawn sky. First quarter, on the other hand, occurs when the moon is high in the sky at sunset, perfect for evening observing.

The moon is so close to us that you don't really need a telescope to study it. Even without any optical aid, the major features of the moon can be clearly seen, if you take the time. In fact, you can see more detail on the moon with your naked eye than you can see on any of the planets with a powerful telescope.

When you look at the first quarter moon, you are seeing a sphere lit by the sun from the right side (in the northern hemisphere; left side in the southern hemisphere). The surface of the moon varies in reflectivity, so that you see a pattern of light and dark. The lighter areas are the older mountainous regions, mostly on the southern half of the moon; the darker areas are younger lava flows, mostly in the north.

Early observers mistook these for seas and oceans, and named them accordingly. Later astronomers realized that the moon is an airless waterless body, and that its “seas” are dryer than the driest deserts on Earth. The water which has been discovered recently on the moon is buried deep beneath the surface.

If you examine the moon more closely in binoculars or a telescope, your attention will be drawn to the “terminator,” the narrow band down the middle of the moon where dark meets light. This is where the sun is rising.

Ordinary binoculars can give a surprisingly detailed view of the moon, especially if they are steadied by mounting them on a tripod. Many binoculars have a tripod socket hidden under a small cap in the hinge between their two halves, but you will need a small L-shaped adapter to connect this to a standard camera tripod. You will be amazed at the improvement a tripod mount makes to the view.

The 10x50 size binocular is the most popular among astronomers. Its 10 times magnification and 50mm objective lenses provide excellent views of both nearby objects like the moon and distant objects like star clusters and galaxies. In recent years more powerful binoculars have become widely available at amazingly low prices. The 15x70 size gives particularly good views of the moon, but must be mounted on a tripod.

Because the sun is rising over the terminator, even the slightest variation in topography is exaggerated by the rising sun. Look especially for craters and isolated mountains on the plains. With the extra magnification afforded by a small telescope, you can actually watch the sun rise over craters in real time.

With a good map of the moon you can become familiar with the “geography” of the moon. More than a thousand craters bear the names of famous astronomers of the past.

Neptune reaches Opposition

This week, on August 27, the planet Neptune reaches opposition.

On August 27 Neptune reaches opposition in Aquarius, making it visible all night. Credit: Starry Night Software

When a planet is in opposition, it lies directly opposite the sun in Earth’s sky. It is highest in the sky when the sun is lowest, which is local midnight. When Daylight Saving Time is in effect, this is close to 1 a.m. local time.

Because Neptune is directly opposite the sun. it rises at the same time as the sun sets, and sets at the same time as the sun rises, so is visible all night.

Now that Pluto has been demoted to “dwarf planet” status, Neptune is the most recent planet to be discovered, on September 23, 1846, and the farthest planet from the sun, at an average of 2,798,310,157 miles (4,503,443,661 km.)

As planets go, Neptune is extremely dim, requiring at least a binocular to become visible. Even in a powerful telescope, it is a tiny blue-green disk with no detail to be seen. Unless you look carefully, you could easily mistake it for a star.

In fact, that happened several times before Neptune’s official discovery. Most famously, Galileo twice observed Neptune while studying Jupiter’s moons in 1612 and 1613, but mistook it for a star both times.

After the discovery of Uranus by William Herschel in 1781, mathematicians calculated the possible location of another planet farther from the sun, but no one looked seriously for it until 1846. The first to actually spot it was German astronomer Johann Gottfried Galle on September 23.

Because it takes Neptune 164 years to circle the sun, it spends an awfully long time in any one constellation. For example, Neptune has been in Aquarius since January 24 2011 and won’t move on into Pisces until May 22 2022. In fact, it has only just completed its first trip around the sun since its discovery, and is again very close to the spot where it was discovered.

Since Neptune is so far from the Earth, it presents too small a disk to be studied well with even the largest telescopes. The only good view we’ve ever gotten of Neptune was in 1989 when the Voyager 2 probe passed within 2740 miles (4400 km.) of Neptune’s cloud tops. At that time it recorded two large blue spots in Neptune’s atmosphere, apparently similar to the Great Red Spot on Jupiter. Voyager 2 also confirmed the existence of a faint ring around Neptune, a ghostly echo of Saturn’s ring system.

The Summer Triangle

Have you ever wished you knew more about the stars overhead? It’s easier than you think.

Lie back on a warm summer night and look straight up. You’ll see three bright stars: Vega, Deneb, and Altair. These mark the corners of the “Summer Triangle” and are your guides to the three constellations of Lyra, Cygnus, and Aquila. Credit: Starry Night Software

All you need to do is lie back on a warm summer evening and look up towards the zenith. It will help if you can find a spot free from light pollution on a night when the Moon isn’t in the sky.

The first thing you will notice is that some stars are brighter than others. The brightest stars are said to be “of first magnitude” and there are three that should leap out at you. The brightest is Vega, almost directly overhead at 10:30 p.m. Daylight Time this week. Next brightest is Altair, down towards the southern horizon, and third is Deneb, off towards the northeast. These three form the “Summer Triangle” and are as characteristic of the summer sky as Orion is of the winter sky.

Since these three stars appear to be about the same brightness, you might think that they were all about the same distance away, but you’d be wrong. Stars come in many colors and brightnesses, and sometimes a very distant, very bright star can look as bright as a very close, relatively dim star. Vega and Altair are both relatively close to the Sun, 25 and 17 light years away, but Deneb, which rivals them in brightness, is a whopping 3300 light years away, making it one of the farthest objects you can see with your unaided eye. In fact, Deneb is an absolutely brilliant star, but so far away that its brightness is greatly dimmed by distance.

As you continue to watch the sky, you may begin to see patterns in the fainter stars. The human brain always tries to find patterns in random shapes, as when we look for the shapes of animals in the clouds. The same is true for stars.

Stars are pretty much randomly distributed across the sky, but from time immemorial humans have grouped them into patterns which we call constellations. Each of the three stars in the Summer Triangle is a member of such a constellation.

If you look closely at Vega, for example, you may notice a small parallelogram of stars just to its south. This reminded ancient astronomers of the musical instrument the lyre, so they named this group of stars Lyra. If you have binoculars, use them to take a closer look at Vega and the stars nearby. Even a small binocular is enough to confirm one of Galileo’s first discoveries when he turned his telescope on the sky: many stars which appear single to the naked eye turn out to be double or multiple with a bit of magnification. Several of the stars near Vega are obvious doubles, even with only 6 or 7 times magnification.

Now take a closer look at Deneb. It stands at one end of a chain of bright stars stretching to the south of Vega. There is a second shorter chain of stars which crosses the first chain at right angles. Different cultures have seen different patterns. The ancient Greeks saw this as a swan and named it Cygnus. Deneb is the tail of the swan, the short chain marks the swan’s wings, and the long chain its outstretched neck, with a brighter star Albireo at the head. Others see this as a Christian cross, and call it the Northern Cross, to distinguish it from Crux, the Southern Cross.

If you are under a dark country sky, you will see that the swan is flying along the faint silvery Milky Way. This is the glow from millions of distant stars, too far away for the individual points of light to be resolved. This was another of Galileo’s discoveries.

Altair marks the head of a different bird: Aquila the Eagle. This has broader wings than the swan, and a distinctive curved tail.

Look for some of the smaller constellations in this part of the sky, in particular Delphinus the Dolphin, one of the few constellations which actually resembles its namesake.

Don’t be dismayed if you can’t readily see these patterns. Sometimes the objects the ancients saw in the sky owed more to imagination than to reality.

Lunar Encounters

What’s that bright “star” next to the moon?

What’s that “star” next to the moon? Over the course of the next month, the moon will pass close to three bright “stars,” which are actually planets, starting with Venus on Friday August 9.  Image Credit: Starry Night software

As the moon moves around the Earth in its orbit, it passes close to many more distant objects in the night sky. Because the moon is nearby and in full sunlight, its light often swamps the objects close to it. Only a few objects are bright enough to show up when close to the moon.

The brightest objects in the sky, other than the sun and moon, are the planets. They follow the same path across the sky as the moon, called the ecliptic because that’s where eclipses take place. So encounters between the moon and planets are very common.

This month, the moon will pass close to three of the brightest planets: Venus, Saturn, and Jupiter.

First up is Venus, this Friday, August 9, just after sunset. Because the ecliptic makes a very shallow angle with the horizon at this time of year, both Venus and the moon will be low in the southwestern sky, so make sure you have a low horizon in this direction. The moon will be only three days old, so will present a very narrow crescent. Be sure to look for “the old moon in the new moon’s arms”: the full disk of the moon lit by the ghostly light reflected from the Earth.

Three nights later, on Monday August 12, the moon will have moved on so that it is framed between the planet Saturn, to the left, and the bright star Spica, to the right. If you have a telescope, take a good look at Saturn, as it will soon be vanishing behind the sun.

The moon’s final encounter for the month won’t be until the morning of Saturday August 31. On that morning early risers will be treated to the waning crescent moon just to the right of the planet Jupiter.

With the moon appearing close to three different bright planets over a span of a few weeks, it’s easy to see how some people, who just happened to look at the moon on these three nights, would be convinced that there is always a “star” next to the moon. In reality, the moon is passing close to three different bright objects located far apart in the sky.

Three Ladies of the Night

Have you ever seen an asteroid? These bodies, though small in size, are very numerous, but very few amateur astronomers have ever seen one. The next couple of weeks give an opportunity to view three asteroids in one night: Flora, Juno, and Iris.

The next couple of weeks provide a great opportunity to track down three of the brightest asteroids. Image Credit: Starry Night software

These asteroids were among the first eight to be discovered. Juno was the third asteroid discovered, in 1804, very soon after Ceres in 1801 and Pallas in 1802. It was discovered by German astronomer Karl Ludwig Harding, and named after Juno, the highest of the Roman goddesses.

At present Juno is located in the western part of the constellation Aquarius, and is about magnitude 8.6. It is too faint to be seen with the unaided eye, but can be easily seen in binoculars or a small telescope. In amateur telescopes it is too small to look any different than a star, hence the name "asteroid" meaning "star-like."

The first asteroids were discovered quite by accident. In 1846, British astronomer John Russell Hind decided to change that by beginning a systematic effort to search for asteroids using the newly published Berlin star maps. He plotted objects along the ecliptic, adding stars that were too faint to be on the maps, and looking for any star that seemed to move. He was rewarded the next year by his discovery of two new asteroids, Iris on August 13 and Flora on October 18.

Iris is named after the Greek goddess of the rainbow and Flora after the Roman goddess of flowers. Juno is the largest of the three, 145 miles (233 km.) in diameter, and Flora the smallest at 80 miles (128 km.), with Iris in between at 125 miles (200 km.) As you can see, they are all quite tiny bodies. Juno has been photographed by the Hooker Telescope at Mount Wilson, and is a rough potato shape with a very large crater on its surface.

Image Credit: Hooker Telescope at Mount Wilson

Currently Iris is in central Aquarius at magnitude 7.6, making it the brightest of the three, and Flora is in Sagittarius at magnitude 8.6. Iris is just south of the globular cluster Messier 2, Juno just north of Messiers 72 and 73, and Flora just west of globular cluster Messier 75.

If you want to replicate the methods used by these early 19th century astronomers, you could plot the "stars" in the neighborhoods indicated on successive nights, and then look for the ones that move. Or you could use the more modern approach of printing finder charts using a modern planetarium program on your computer. All three are moving slowly westward, from left to right for observers in the northern hemisphere.

It’s really quite remarkable that we can see these tiny chunks of rock, farther away than Mars, with ordinary binoculars. Don’t miss this chance.

Spot Mercury at Dawn

Most skygazers have never seen the planet Mercury because it never strays very far from the sun. This week is one of the rare opportunities to catch Mercury just before sunrise.

On the morning of Tuesday July 30, Mercury will be at its furthest from the sun, just under Jupiter and Mars. Credit: Starry Night software

Two things about Mercury make it a tough target. First and foremost, it never strays far from the sun because of its location in the inner solar system. It is always observed against a bright twilight sky, either half an hour after sunset or half an hour before sunrise, and always low in the sky.

Secondly, it is a tiny object. Now that Pluto has been demoted to "dwarf planet" status, Mercury, at 1550 miles (2440 km.) in diameter, is the smallest of the eight planets, 38 percent of Earth's diameter.

The challenge in observing Mercury is to find a tiny speck of light, low in the sky, against bright wilight. This week offers one of the rare opportunities when conditions are at their best.

Mercury will be farthest from the sun in our sky on July 30. The weeks just before and after this date are equally favourable.

The best time will be about 40 minutes before sunrise, a balance between Mercury's altitude above the horizon and the brightness of twilight. Scanning the sky with binoculars will help to spot the tiny speck of light. Once spotted in binoculars, you should just be able to see Mercury with the unaided eye.

Because of Mercury's low altitude, a low cloudless eastern horizon is necessary. The task is made easier by the presence of two much brighter planets in the dawn sky, Jupiter and Mars. Mercury will be below and to the left of these two objects.

Once you spot Mercury, locate it relative to landmarks on your horizon, and see how high you can follow it as it rises and the sky becomes brighter. If you have a telescope, this task becomes easier.

Seen in a telescope, Mercury will look like a tiny gibbous moon. As it rises higher in the sky, it will clear the turbulence, and the view will improve.

Congratulations...you have succeeded in observing one of the most elusive objects in the sky. It is said that the great 16th century astronomer Johannes Kepler never managed to see Mercury.

More Close Encounters

This month is full of encounters between astronomical objects. Last week we had the Moon and Spica in Virgo in the evening, and Mars and Messier 35 in Gemini in the morning, and this week we have Venus and Regulus in Leo in the evening, and Mars and Jupiter in Gemini in the morning.

On the night of Sunday July 21, the planet Venus passes just north of the bright star Regulus in Leo in the western sky just after sunset.  Credit: Starry Night software

Although Regulus is one of the brightest stars in the sky, Venus is far brighter, and will be by far the easier to see. Once you spot Venus, look below and to its left to spot Regulus. If you’re south of the equator, Regulus will be above and to the left of Venus.

Later that night, an hour before dawn, look in the northeastern sky to see Jupiter and Mars rise together. Jupiter will be the brighter of the two, with reddish Mars above and to its left. If you’re in the southern hemisphere, Mars will be below and to the left of Jupiter.

Later that night, an hour before sunrise, the planet Mars passes Jupiter.  Credit: Starry Night software

If you have access to a telescope, these twin planets will make an interesting sight. Jupiter will be accompanied by its four bright moons, Io, Europa, Ganymede, and Callisto. What will be most noticeable is the tremendous difference in size between the two planets. Both are on the far side of the Sun, but still Mars is much closer than Jupiter. Even so, Jupiter will appear more than eight times larger in diameter than Mars.

The planets move relatively slowly as compared to the moon, so that, if you miss these events on Sunday night and Monday morning, try again the following night and morning, and these encounters will still be close enough to be fascinating.

The Milky Way Galaxy

When people ask me what I do, I tell them I study galaxy mergers and evolution. Sometimes they will nod, smile and listen for awhile, and then more often than not, the question will come:  "What exactly is a galaxy?".  I then realize that most people are never taught in school what a galaxy is, and I certainly wasn't before I went to study astronomy at university.  If you are lucky enough to be under a dark, clear sky on a summers night, understanding what a galaxy is becomes so much easier!  (or alternatively, fire up Starry Night at home :) ).

We live within a fairly large spiral disk galaxy, about two thirds from the centre, in one of its spiral arms. During the summer, a faint cloud-like band, known as the Milky Way, arches across the night sky.  If you were in space, far above the Earth, you would see that this band of light actually goes all the way around the sky, forming a great circle around us.  This is because we are inside of the disk of the Milky Way, and the band of light is actually composed of parts of the galaxy which are very far away.  With a telescope, you can see that the Milky Way is composed of millions of individual stars, but to the unaided eye, they all blur together to form a cloud.  You might notice that there are gaps and dark spots in the Milky way.  These are actually massive clouds of gas and dust which block the light from the stars beyond them.

Most of the stars we see in the sky are part of the spiral arm our solar system is located in, known as the Orion arm.  The stars you can see with your naked eyes are like houses in our neighbourhood, while the stars in the other arms are like distant parts of a massive city.  The centre of the Milky Way galaxy is located in the direction of Sagittarius, where a super massive black hole lives.  That black hole is the centre of our galaxy, with all the stars orbiting around it, just like our Sun is the centre of our solar system with the planets orbiting around it, but on a much, much larger scale.  What is a black hole you ask?  We'll look at that in my next post!  For now, make sure you find the time this summer to get out of the city, to a place free from light pollution, and enjoy the view from Earth of our amazing galaxy, the Milky Way!

Close Encounters in the July Sky

If you only glance at the sky now and then, you may think it's pretty much the same every night. Regular sky watchers know better, and are aware of the constant changes the sky undergoes.

On the night of Monday July 15, the first quarter moon sails just north of the bright star Spica in Virgo.  Credit: Starry Night software

Take the moon, for example. Although most people know that the moon revolves around the Earth, they may never have noticed this movement with their own eyes. This week we get a chance to do exactly that.

The problem is that much of the time the moon is in an open reach of sky, and there are no fixed points to mark its movement. This week it passes close to the bright star Spica and the planet Saturn in the constellation Virgo.

On Sunday night, July 14, the moon will be well to the right of Spica (in the northern hemisphere...reverse left and right if you're south of the equator) in the evening sky.

On Monday night, July 15, the moon will be immediately above Spica. In fact, if you live in Central America or Africa, the moon will pass right in front of Spica, in what is called a lunar occultation.

On Tuesday night, July 16, the moon will have moved well to the left of the moon, and will be just below the planet Saturn.

On Wednesday night, July 17, the moon will be well to the left of Saturn.

Let's go back to Monday night, when the moon is closest to Spica. This will not be the only astronomical encounter this night. Stay up until the wee hours of the morning, and you'll see Mars and Jupiter rise an hour before sunrise. If you look at Mars in binoculars or a small telescope, you will see that it is close to one of the brightest star clusters in the sky, Messier 35 in Gemini.

Later that night, an hour before sunrise, the planet Mars passes the open cluster Messier 35 in Gemini. Jupiter is approaching from the east.  Credit: Starry Night software

Now we will see how the planets move as well. Because they are farther away, they seem to move much more slowly than the moon. A few months ago we were watching Mars and Jupiter fade into evening twilight as they moved behind the sun. Now they have emerged into morning twilight to begin their next apparition in our night sky. If you're an early riser, keep an eye on them over the next couple of weeks as they approach each other, pass on the morning of July 22, and then draw apart again.