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.

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 youve 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 Earths 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 suns 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.

Seen from far above the suns 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.

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. Jupiters 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

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

Jupiters moons are very small, even in a large telescope, but their shadows are slightly larger, and can often be seen crossing Jupiters face with a good amateur telescope. Ive 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 Jupiters usual four moons are visible. Thats because two of the Moons, Io and Callisto, are in front of Jupiters disk, and are said to be in transit. You probably wont 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 Jupiters bright clouds. I usually see it as a tiny greyish spot. Look more closely, and youll see two small dark shadows on Jupiters face. One of these is Ios shadow, but the other is not the shadow of Callisto. Instead it is the shadow of Ganymede, off to Jupiters right. Thats because of the angle at which the sun is illuminating the tableau. 

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

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

Nearly two hours later at 9:55 p.m., Ganymedes shadow has left, and Ios 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. Its more usually seen as a light notch in the North edge of Jupiters 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. Ganymedes shadow starts across Jupiters face at 8:58 p.m. EDT. Ios shadow follows at 10:01 p.m., and both shadows are present until Ios shadow leaves at 12:18 a.m., followed by Ganymedes at 12:34 a.m. 

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., Ios faster moving shadow actually passes Ganymedes, and the two shadows merge.

At 11:48 p.m., Ios faster moving shadow catches up with Ganymedes, 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.

If you'd like to follow along with NASA's New Horizons Mission to Pluto and the Kuiper Belt, please download our FREE Pluto Safari app.  It is available for iOS and Android mobile devices. Simulate the July 14, 2015 flyby of Pluto, get regular mission news updates, and learn the history of Pluto.

Simulation Curriculum is the leader in space science curriculum solutions and the makers of Starry Night, SkySafari and Pluto Safari. Follow the mission to Pluto with us on Twitter @SkySafariAstro, Facebook and Instagram