Extreme Venus And A Close Encounter With Jupiter

On Monday morning, October 26, Venus will be at its greatest elongation west, the farthest it can get from the sun as seen from Earth.

Venus has been dominating the morning sky for the last two months, and reaches its greatest angular separation from the sun on the 26th: 46 degrees west of the sun. Remember, directions in the sky are the reverse of directions on the surface of the Earth because we are looking outward rather than inward, so west is to the right in the sky if you live in the northern hemisphere.

The inner planets, Mercury and Venus, never stray far from the sun in Earth’s sky. Mercury was at greatest elongation west, 18 degrees from the sun, last Friday, October 16, and is now moving back towards the sun. Venus is still moving away from the sun, but will pause and reverse directions on the 26th. Mars and Jupiter, with orbits outside that of Earth, both appear to be moving steadily westward relative to the sun.

As a result, Venus and Jupiter will pass very close to each other on the 26th, within 1.1 degrees.

Venus is by far the brightest of the two, at magnitude –4.6, while Jupiter is nearly 3 magnitudes fainter at magnitude –1.8. Mars, which is also nearby in the sky, comes a distant third in brightness at magnitude +1.7, more than 3 magnitudes fainter than Jupiter and 6 magnitudes fainter than Venus.

This close conjunction of Venus, Jupiter, and Mars is best observed with the naked eye or binoculars. In a telescope, Venus will appear as a perfect miniature quarter moon, because at greatest elongation it is lit by the sun exactly from its left side. It is 24 arc seconds in diameter, slightly smaller than Jupiter at 33 arc seconds, but much larger than Mars at 4 arc seconds.

Venus appears brightest because it is both close to the sun and close to Earth. Jupiter is much larger in actual diameter, but is far from the sun so only lit dimly. Mars, in between in distance from the sun and small in size, is currently very far from Earth, so appears even dimmer.

To put them in perspective, even though grouped close together in our sky, the three planets are at vastly different distances. Venus is closest at 0.69 astronomical units, Mars next at 2.2 a.u., and Jupiter farthest at 6.0 a.u., nearly ten times farther than Venus. An astronomical unit is the average distance between Earth and the sun.

Many photographers have been taking advantage of the close approach of these three planets to photograph them in the dawn sky, though the different brightnesses present a challenge in exposure. As always, we welcome your pictures of this beautiful event.

Planet Adventures In The Sky

Sometimes a whole bunch of different sky events happen over a very short period, giving skywatchers a chance to witness many unusual events in a few days. The next week or two is a case in point.

Uranus at opposition

First, the planet Uranus is just past opposition, so it is visible all night long. It’s also in a location where you can spot its dim glow rather easily. You will need a binocular to bring out the faint planet from its surrounding stars

Look at the chart above, and you will see how to spot Uranus using an easy bit of star hopping.

Start with visualizing an equilateral triangle with one side marked by Hamal in Aries and Algenib, the lower left star of the square of Pegasus. The third point of the triangle is Alrescha, Alpha Piscium.

Although only fourth magnitude, Alrescha is the second brightest star in the dim constellation of Pisces. It marks the knot which joins the two chains of stars attached to the two fish of Pisces. Follow the lower chain of stars to the right under the square of Pegasus to find Uranus.

The stars that make up this chain do not have names, only Greek letters. Starting at Alrescha, they are Nu, Mu, Zeta, and Epsilon. You don’t need to know their names, just look for Uranus between the last two, a bit to the south of the line joining them. 

In binoculars, Uranus will look like a blue green 6th magnitude star. In a powerful telescope, it will show as a tiny blue-green disk 3.7 arc seconds in diameter.

Arc seconds

Planetary diameters are usually measured in arc seconds: the number of seconds they subtend at the observer’s eye. There are 360 degrees in a circle, 60 minutes in a degree, and 60 seconds in a minute. To give you some idea how tiny the planets appear, the sun and moon are both around 30 arc minutes in diameter, or 1800 arc seconds. The largest any planet can appear to Earthbound astronomers is Venus at its closest, which is about 1 arc minute or 60 arc seconds in diameter. Thus all the planets are typically smaller in a telescope than the largest craters on the moon. 

Mercury at greatest elongation

On Thursday, October 15, Mercury will be at the farthest it strays west of the sun. Because it is only 18 degrees away from the sun, it can best be seen about half an hour before sunrise. Again it is easiest to spot in binoculars. It is about twice the angular diameter of Uranus, so will look like a tiny half moon 8 arc seconds across in a large telescope.

Mars near Jupiter

On Sunday morning, October 18, Mars will be in conjunction with Jupiter, and the two planets will be close enough together that both will fit in the same field of a small telescope. Look for Mars just above Jupiter.  

Both will be on the far side of the Sun, so much smaller than they usually appear when in the evening sky. Jupiter measures 32 arc seconds while Mars is only 4.1 arc seconds, half the size of Mercury. By the time they reach opposition in spring of 2016, they will swell to 44 and 18.3 arc seconds respectively.

Double shadow transits on Jupiter 

Speaking of Jupiter, there will be two interesting passages of the shadows of its moons across its face in the next two weeks. These require at least a 5-inch telescope and steady observing conditions.

Again on Sunday morning, October 18, the shadow of the moon Io will chase the shadow of Ganymede across Jupiter’s face. Both shadows will be on Jupiter from 6:42 to 8:08 a.m. EDT, but will be increasingly hard to see after sunrise at around 7:30 a.m. local time. Observers in western parts of North America will have a better view because the transits will occur between 3:42 and 5:08 a.m. PDT, long before sunrise, though Jupiter will be lower in the sky.

Exactly a week later. on Sunday October 25, there will be another double shadow transit. This one is especially interesting because both shadows enter the disk at almost the same time, 8:36 a.m. EDT or 5:36 a.m. PDT, but then cross at very different speeds, Io’s shadow taking 2 hours and 17 minutes to cross while Ganymede’s takes 3 and a half hours, because of its greater distance from Jupiter and larger orbit diameter.

This transit occurs in full daylight in eastern North America, so probably won’t be visible, but is at a much more favorable time for western observers.

All in all, planetary observers will have a busy week ahead and, as always, we welcome your images of these events.

Triple Conjunction at Sunset Between The Planets Mercury, Jupiter And The Bright Star Regulus

Over the next few nights, observers with low western horizons may be able to observe a triple conjunction between the planets Mercury and Jupiter and the bright star Regulus.

On Friday, August 7, the two planets and star will form an equilateral triangle, 1 degree on a side. Mercury will be on top, Regulus to the left, and Jupiter to the right.

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Look for them with binoculars just after sunset. Jupiter will be in the middle and the brightest of the three, at magnitude –1.7. Mercury will be on the right at magnitude –0.7. Regulus will be the faintest of the three, on the left at magnitude +1.3.

Brightness in astronomy is measured on an upside-down scale where the faintest stars visible to the naked eye are magnitude 6, the brightest stars are around magnitude 0, and really bright objects like the sun, moon, and planets have negative magnitudes.

All three of these objects will be very bright, but will be masked by the brightness of the sky just after sunset. If you wait until the sky has darkened, they will probably be too low to observe.

Three nights later, on Monday, August 10, Mercury will have moved away to the left, but Regulus and Jupiter will have moved closer to each other, less than half a degree apart. This conjunction will be very hard to observe because of its low altitude.

This will probably be your last chance to observe Jupiter before it passes behind the Sun on August 26. Look for it in the morning sky in mid-September.

Venus and Jupiter, Up Close and Personal

If you’ve been watching the western sky just after sunset lately, you will have noticed two bright objects, gradually drawing closer.

These are the two brightest planets, Venus and Jupiter. Venus is the brighter of the two, currently magnitude –4.6 on the upside-down brightness scale astronomers use. It will get slightly brighter over the next 10 days, reaching greatest brilliancy on July 10 at magnitude –4.7.

Jupiter is somewhat fainter at magnitude –1.8, down from a maximum of –2.6 when it was in opposition on February 6.

Although the two planets look very close in Earth’s sky, they are in fact very far apart, on opposite sides of the sun. The first graphic shows their true positions, as seen from far above the sun’s north pole. Venus is slightly nearer Earth than the sun, 0.512 astronomical units distant (47.6 million miles or 76.5 million km.) while Jupiter is 6.083 astronomical units away (565 million miles or 910 million km.) on the far side of the sun.

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Seen from far above the sun’s north pole on Wednesday, July 1, Earth, Venus and Jupiter lie almost on a perfect straight line.  Credit: Starry Night software.

Viewed in a telescope, the two planets are, by coincidence, exactly the same apparent size, 32 arc seconds in diameter, about a 60th of the apparent diameter of the moon, but look very different.

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Seen in the eyepiece of a telescope magnifying 65 times at sunset on July 1, Venus and Jupiter appear very close, but look very different, even though both are the same angular size. Credit: Starry Night software.

Venus, with its bright cloud cover and closeness to the sun, is a brilliant white crescent, lit from slightly behind because it is moving between us and the sun. Jupiter’s cloud tops are somewhat darker than Venus’, but it is also more than seven times farther away from the sun. As a result, despite its large size, Jupiter appears much fainter in a telescope than Venus.

Two bright planets in close proximity make a striking sight with the naked eye. In binoculars you should be able to see that Venus is a tiny crescent and Jupiter is a disk accompanied by 3 tiny moons (on July 1, Callisto will be behind Jupiter). A small telescope will make the view much clearer.

As you continue to watch these two planets over the next few weeks you will see them draw apart as both get closer to the sun, Venus passing between us and the sun on August 15, and Jupiter passing behind the sun on August 26. In another month, both will reappear in the morning sky, where they will join Mars, which passed behind the sun on June 14.

SkyWatching: Shadow Play on Jupiter

Jupiter’s rapidly moving moons constantly surprise us with their dance around the giant planet. There will be two spectacular shadow plays this week.

Jupiter’s moons are very small, even in a large telescope, but their shadows are slightly larger, and can often be seen crossing Jupiter’s face with a good amateur telescope. I’ve seen the shadows with telescopes as small as 90mm aperture, but a telescope with 6-inch or larger aperture will show them much more clearly. Steady atmospheric seeing is also essential.

If you live on the eastern seaboard, look for Jupiter just after sunset on Wednesday, May 20 around 8:10 p.m. EDT. The first thing you will notice is that only two of Jupiter’s usual four moons are visible. That’s because two of the Moons, Io and Callisto, are in front of Jupiter’s disk, and are said to be “in transit.” You probably won’t see Io, because its color and brightness blend in so well with the cloud tops behind it. You may be able to see Callisto because its dark surface stands out against Jupiter’s bright clouds. I usually see it as a tiny greyish spot. Look more closely, and you’ll see two small dark shadows on Jupiter’s face. One of these is Io’s shadow, but the other is not the shadow of Callisto. Instead it is the shadow of Ganymede, off to Jupiter’s right. That’s because of the angle at which the sun is illuminating the tableau. 

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On Wednesday night, May 20, the shadows of Jupiter’s moons Ganymede and Io will cross Jupiter’s face. This shows the shadows at 8:10 p.m. EDT, just after Io’s shadow has started across, and just before Ganymede’s shadow leaves.  Credit: Starry Night software.

Take another look later in the evening, around 9:55 p.m., and you’ll see that Ganymede’s shadow has left the disk and that Io’s shadow is about to be hidden behind Callisto. This will be the first time I have ever seen a moon’s shadow eclipsed by another moon. 

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Nearly two hours later at 9:55 p.m., Ganymede’s shadow has left, and Io’s shadow is about to be eclipsed by the moon Callisto. The Great Red Spot is well placed close to Jupiter’s central meridian. Credit: Starry Night software.

Also keep a lookout for the Great Red Spot, though it is not nearly as “great” nor as “red” as it once was. It’s more usually seen as a light notch in the North edge of Jupiter’s South Equatorial Belt. At moments of steady seeing, its salmon pink color may appear briefly.

A week later on May 27, the situation nearly repeats itself, but is about two hours later, making it more easily seen across the whole of North America. Ganymede’s shadow starts across Jupiter’s face at 8:58 p.m. EDT. Io’s shadow follows at 10:01 p.m., and both shadows are present until Io’s shadow leaves at 12:18 a.m., followed by Ganymede’s at 12:34 a.m. 

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Exactly a week later, on Wednesday, May 27 at 10:05 p.m., the pattern repeats, except that the shadows are closer together and Callisto is no longer in front of Jupiter. Credit: Starry Night software.

Notice that at around 11:48 p.m., Io’s faster moving shadow actually passes Ganymede’s, and the two shadows merge.

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At 11:48 p.m., Io’s faster moving shadow catches up with Ganymede’s, and the two shadows merge. Again, the Great Red Spot is well placed. Credit: Starry Night software.

Once again, the Great Red Spot should be in evidence.

If you live west of the Eastern time zone, be sure to subtract the appropriate corrections from the times given above: 1 hour for CDT, 2 hours for MDT, and 3 hours for PDT.

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