On May 09, 2016, amateur astronomers with small telescopes and solar filters will be able to observe Mercury’s tiny silhouette moving slowly across the face of the Sun. Astronomers call this event a “transit”.Read More
On Friday, January 8, the planet Venus will appear to pass just 5 arc minutes north of the planet Saturn, That is a mere one-sixth of the diameter of the moon, a small enough distance to fit in the eyepiece of a powerful telescope.
This will be a rare opportunity to see two planets at the same time in a telescope’s narrow field of view. In a lifetime of observing the skies, I have seen such a close conjunction of two planets only two or three times. With the naked eye, sharp-eyed observers will be hard pressed to separate the two points of light.
Unfortunately for observers in North America, the point of closest conjunction will occur at 11 p.m. EST, while the planets are below the local horizon. For a skywatcher in New York, for example, the planets won’t clear the eastern horizon until 5 a.m. EST, at which time they will have separated so that they are 17 arc minutes apart, or slightly more than half the diameter of the moon. Even so, they will still fit in a telescope eyepiece.
Of course, this conjunction is something of an optical illusion. The two planets aren’t anywhere near each other in space, but merely appear close together from our perspective here on planet Earth.
Venus is currently on the far side of the Sun from Earth, 1.22 astronomical units distant (1.22 times the average distance from the Earth to the sun), so it appears similar to a gibbous moon.
Saturn is 10.79 astronomical units from Earth, nearly 9 times farther away than Saturn.
The two appear almost the same angular diameter: 14 arc seconds for Venus, 15 arc seconds for Saturn, yet in reality Saturn is actually almost 10 times the diameter of Venus. So Saturn’s greater distance balances out is larger size, and the two appear almost the same from Earth.
The most striking difference between the two is their difference in brightness. Venus is magnitude –4.0 on the upside-down brightness scale astronomers use, while Saturn is only magnitude +0.5, 4.5 magnitudes (or almost 100 times) fainter than Venus. This difference is mainly due to Venus’ closeness to the sun (0.72 astronomical units) compared to Saturn’s (9.55 astronomical units).
When you look for the twin planets just before dawn on Saturday morning, the first thing you will notice is Venus shining brightly in the southeastern sky. You will have to look closely to spot “tiny” Saturn just above and to the right of it.
If you own a planetarium program like Starry Night that lets you travel to other planets, check out the view Saturday morning from Saturn. You will see Earth and Venus in a close conjunction, Venus a narrow crescent from Saturn’s perspective, and Earth a rounded gibbous shape, since despite being close in the sky, they are actually on opposite sides of the sun, and lit by it quite differently.
Next Tuesday or Wednesday marks a major turning point in the annual cycle of the seasons. The sun reaches its southernmost position in the sky, resulting in the shortest day of the year in the northern hemisphere, and the longest day of the year in the southern hemisphere.
The solstice gets its name from the apparent stop (“stice”) in the motion of the sun (“sol”). This was carefully recorded by the earliest astronomers; monuments like Stonehenge are thought to have been used to mark the extreme positions of the sun in the sky. The December solstice has long marked the beginning of the new year, and it’s mainly because of slippage in our calendar that it now occurs eleven days before the “official” New Year, January 1.
Solstice day is a day of celebration in many cultures. The Romans knew this as “Saturnalia,” and the early Christians adopted this date to mark the birth of Christ, so that they could celebrate without drawing the attention of their Roman masters.
You’ll notice I said, “Tuesday or Wednesday.” That’s because, although the time of solstice is exactly the same everywhere in the world, because of our local clocks it falls on different days in different places. The exact time of solstice this year is December 22 04:48 coordinated universal time, the time used by astronomers and pilots everywhere.
In England, where the prime meridian lies, the solstice will occur a 4:48 a.m. GMT on Wednesday, December 22. Similarly, it will occur in Europe and Africa in the early hours of Wednesday morning.
In North America, we subtract a number of hours from UTC to get our local times. In most of eastern North America, we are on Eastern Standard Time, and subtract 5 hours, so the solstice falls at 11:48 p.m. on the previous day, Tuesday, December 21. The farther west we go in North America, the earlier the solstice occurs in the evening, so that on the Pacific coast it occurs at 8:48 p.m. PST.
Remember, these are all exactly the same time in the broader scheme of things; local times are just vagaries of the way we handle time around the world.
The graphic shows the sky as it would appear at the time of solstice from a location where the solstice occurs at noon, which this year would be Vietnam. Through the magic of Starry Night software, we have turned the blue sky transparent and have added the two coordinate systems we use to mark positions in the sky.
The red line across the sky marks the celestial equator, half way between the celestial north and south poles. It shows 18h on the meridian because we measure right ascension, the celestial equivalent of longitude, from the vernal equinox, exactly 9 months ago.
The green line marks the ecliptic, the path that the sun appears to follow across the sky. The sun is at its southernmost position, at a declination (celestial equivalent of latitude) of exactly 23 degrees and 26 minutes, which just happens to be the exact angle at which the Earth’s poles are tilted from the ecliptic.
The height of the sun above the horizon on solstice day depends on your latitude on the surface of the Earth. Where I live in southern Canada, it hangs very low in the southern sky, barely 22 degrees above the horizon. For anyone north of the Arctic Circle, it never rises at all. Even in the southernmost continental United States, it’s barely 42 degrees above the horizon, less than half way up the sky.
In the southern hemisphere, the situation is reversed. The December solstice marks the longest day in the year, combined with the shortest night. Longer days mean more hours of sunshine and warmer weather.
You may have noticed that I have avoided using the words “winter” and “summer.” That’s because, even though the sun’s position in the sky is responsible for our seasons, it does not match exactly with the seasons as we experience them. That’s because there is a lag of about six weeks between the astronomical season markers, solstices and equinoxes, and the actual seasons.
The Earth is a complex ecological system, and it takes a while for the sun’s movement to effect the temperature. Sometimes the astronomical markers are referred to as “the first day of…” which makes some sense, if the seasons were exactly 3 months long. However, the lengths of the seasons in any particular location tend to vary depending on local conditions. Hence the old joke about the Canadian seasons: eleven months of winter and one month of bad sledding.
Next Monday, December 7, the planet Venus will do a vanishing act behind the moon.
In the course of its monthly journey around Earth, the moon often passes in front of stars, and occasionally planets.
Next Monday it will pass in front of the brightest of them all, the planet Venus, in what is called a lunar occultation.
This will be visible, weather permitting, all over North America. The only catch is that it occurs in broad daylight.
I’m often surprised that many people don’t know that the moon is visible in the daytime sky; in fact my son first spotted it when he was about 2 years old. That mainly says that very few people bother to look closely at the sky. On Monday, the moon will be a slender crescent only a few days away from new moon, so may be a bit harder to spot, though if you start observing at dawn you should have no trouble following it up into the blue sky.
Venus is also visible in daylight provided you know exactly where to look. On Monday, its closeness to the moon will help you spot it.
The time the occultation will occur depends on your exact location. For example, in New York City, Venus will disappear behind the moon at 12:43 p.m. EST and reappear at 1:51 pm. In Chicago the occultation lasts from 11:20 a.m. CST until 12:35 p.m. In Los Angeles, the times are 8:06 and 9:56 a.m. PST. The easiest way to get the exact times for your location is with a planetarium program like Starry Night or SkySafari.
It was observations of occultations by the moon of stars and planets that first told us that the moon has no significant atmosphere. Stars simply wink out and in as they pass behind the moon; planets show no fuzziness as they disappear behind the moon’s edge.
Starry Night gives you the opportunity to travel to other places in the universe, and view events from a different perspective. Our second image today shows the view of this occultation from the planet Venus at 12:35 p.m. EST. Instead of an occultation, we see what is called a transit, as one object (the moon) passes in front of another (Earth). The moon is moving from right to left in this view.
The occultation is already over on the east coast, to the right of the Moon, and has yet to begin on the west coast, to its left. The surface of the moon looks strange because we are seeing its “far side,” the side permanently turned away from Earth. This side has more mountains and craters and fewer open flat plains than the side we usually see.
One of the first things every new moon watcher learns is that, when observing the moon, timing is everything.
You might think that the best time to look at the moon with binoculars or a small telescope is when the moon is full, because it looks so big and bright. Well, that’s usually the worst time of the month for observing the moon. That’s because at full moon, the sun is high overhead on the moon’s surface, making it look like the desert at high noon. Far better times are at first and last quarters, when the sun is shining obliquely across the moon’s terrain, casting mountains and craters into high relief.
That’s what makes this week the perfect time to begin your exploration of our nearest neighbor in space. First quarter will be on Thursday night, November 19, meaning that the moon is one quarter of the way around the Earth in its monthly journey. The light will be hitting the terminator, the boundary between light and shadow, very obliquely, making the moon exactly half illuminated.
Because the moon is a world in its own right almost as large as the planet Mercury, and because it is so close to us, its surface geography is know almost as well as that of the Earth. Its surface has been mapped for hundreds of years, and most of its features have been named. Everyone who studies the moon soon becomes familiar with its major features and their names.
What catches everyone’s attention first at this phase of the moon is the three gigantic craters almost exactly in the centre of the moon. Like almost all the craters on the moon, these are named for famous astronomers of antiquity.
The northernmost and largest of the three is Ptolemaeus, named for Claudius Ptolemy. His name was given pride of place on the moon because he was probably the greatest astronomer of antiquity. His book, the Almagest, was written in the middle of the second century AD, and summarized all of astronomical knowledge of his time. It was written in Greek, but is known by its Arabic title because it only survived into the modern day in an Arabic translation.
The crater Ptolemaeus is 95 miles (153 km) in diameter, and is characterized by a wide flat floor pockmarked by many small craterlets.
The middle crater of these three is Alphonsus, named for Alphonso El Sabio, king of Castile in what is now Spain in the 13th century. Besides being a king, he was also an astronomer and a musician, and compiled the Alphonsine tables which were used to calculate planetary positions until the time of Kepler. This crater is a bit smaller than Ptolemaeus, but is more of a typical lunar crater with a central peak and complex system of rilles on its floor. This crater became famous in the 1950s when a Russian astronomer claimed to have seen a volcanic emission in it.
The third crater Arzachel is smaller still, 60 miles (97 km) in diameter, but with a larger central peak.
Just to the north of these three craters is the broad arc of the Apennine mountain range. This range is 370 miles (600 km) long and has peaks as high as 16,000 feet (5,000 m) in height. At its northern end is Mount Hadley, near which the astronauts of Apollo 15 landed in July 1971.
To the west of the Apennines is the broad expanse of the Mare Imbrium, the so-called “Sea of Rains.” Early astronomers had no idea how dry and airless the moon was, and its “seas” are dryer than the driest deserts on Earth. This is the second largest “mare” on the moon, exceeded only by the Oceanus Procellarum to its south.
The Mare Imbrium is a huge flat plain 720 miles (1160 km) across, with an area of 320,000 square miles (830,000 sq km). It contains many small mountain massifs and individual peaks. The lunar Alps are to its north, enclosing the beautiful crater Plato, 63 miles (101 km) in diameter. Its almost flat floor contains a number of small craterlets which are used by lunar astronomers to test the resolution of their telescopes. How many can you see?
Near the south pole of the moon, look for the gigantic crater Clavius, 140 miles (225 km) in diameter, and containing a whole series of craters in its interior graded in size. The largest of these is large enough to warrant a name of its own, Rutherford, 32 miles (51 km) in diameter.
With these landmarks learned, you can begin your exploration of the moon’s finer details. What I like to do myself is to start at the moon’s north pole and work my way down towards the southern highlands along the terminator, using a good lunar map to identify the topography.
Most of the features I’ve described can be seen with ordinary binoculars. Even the smallest telescope will reveal hundreds of smaller features. The moon is generally a good target for photography, because it is so bright that long exposures are not needed. You can even put your smartphone up to the eyepiece of your telescope and get remarkably good photos that way.
You can start exploring tonight! All you need is a little practical advice and this guide. There’s an infinite universe of celestial wonders to see and discover. Download our FREE Welcome Guide To Astronomy (PDF [643KB])
Now that Pluto has been demoted to a dwarf planet, Mercury is the smallest of the eight planets. With a diameter of 3032 miles (4879 km.), it is slightly more than a third of the diameter of Earth, and smaller than the solar system’s two largest moons, Ganymede and Titan. Because of its tight orbit around the sun, Mercury never strays far into the night sky, peeping tantalizingly over the horizon a few times a year. The next two weeks will be your best chance for seeing Mercury in evening twilight this year.
Timing is the secret for catching sight of Mercury. Try too early, and its tiny speck of light will be lost against the twilight sky. Try too late, and Mercury will be too close to the horizon. I’ve found the best time to be about half an hour after sunset. Binoculars are helpful in initially spotting Mercury, but once located in binoculars you should be able to see it with the unaided eye.
Currently Venus is shining brightly in the evening sky, and it can be a helpful guide to spotting Mercury, about two-thirds of the way down towards the horizon, and slightly to your right. Don’t confuse it with nearby Aldebaran, which will have a noticeably reddish color and will probably twinkle, while Mercury shines with a more steady light.
In a telescope, Mercury is a disappointing sight. Like Venus, Mercury exhibits phases as it passes between us and the sun. At present it is slightly gibbous. On Saturday, May 2, it will look just like a miniature first quarter moon. After that, it will assume a crescent shape.
Because Mercury is always seen close to the horizon, it is a challenge to see its surface markings, even in a powerful telescope. Serious observers of Mercury prefer to observe it in the daytime sky, now relatively easy to do because of computerized telescopes. But always be very careful when observing with the sun above the horizon, because even the briefest view of the sun in a telescope will do permanent harm to your eyes.
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