Jupiter Peaks in Perfection, the Moon Becomes a Pre-dawn Silver Sliver, and some Deep Sky Delights!

(Above: A simulated view of Jupiter for 10:20 pm EDT on May 12, when the moon Galilean Europa and its shadow will be crossing the planet’s disk.)

The Moon and Planets

Tomorrow evening, the moon will reach its Last Quarter phase, the start of the final week of its monthly trip around the Earth. At last quarter, the angle between the Earth, the moon, and the illuminating sun is close to 90°, so observers worldwide will see the moon’s globe half illuminated (on the side facing towards the pre-dawn sun). Last quarter moon rise after midnight and remain visible in the morning daytime sky. For the rest of the week, the moon will wane to an old crescent and shift closer to the sun. Look for its silver sliver just over the eastern horizon next weekend.

(Above: On the weekend of May 12–13, the moon will appear as a slim crescent over the pre-dawn eastern horizon, not far from elusive Mercury. The sky is shown for 5:30 am local time on Saturday.)

This week Venus will continue its domination of the western early evening sky. It will set at about 11 pm local time. Every evening, Venus’ orbital motion will lift it higher. It will end this week sitting between the stars marking the horns of Taurus (the Bull), with Zeta (ζ) Tauri (also known as天关, Tiānguān in Chinese) on the left and Elnath on the right. Elnath is also shared by the adjacent constellation Auriga (the Charioteer). Pull out your telescope and look at Venus. It’s now exhibiting a gibbous phase, appearing somewhat flattened on the top.

On Tuesday, May 8 at 9 pm EDT, Jupiter will be exactly opposite the sun in the sky, and visible all night long. On opposition night, the planet’s disk will be its brightest and largest (44.8 arc-seconds across) for the year — but it will look almost that large throughout May. The king of planets will rise in the east this week after 8 pm local time, will reach its highest elevation (about three fist diameters) above the southern horizon around 1:15 am local time, and then descend into the southwestern sky at sunrise. The bright star sitting just to the upper right of Jupiter is Libra’s (the Scales) brightest star, Zubenelgenubi. The name means “Southern Claw” because it used to be part of nearby Scorpius (the Scorpion).

(Above: On May 8, shown here at 10 pm local time, Jupiter reaches opposition, when it is closest and brightest for the year. The stars of Libra will surround the planet all summer.)

Between Monday evening at 11 pm and Tuesday at 1 am EDT, Jupiter’s innermost moon Io and its little round black shadow will cross (or transit) Jupiter’s disk. On Saturday evening, the moon Europa and its shadow will transit between 8:38 pm to 10:54 pm EDT. On Sunday evening, the moon Ganymede and its shadow will cross near Jupiter’s north pole between 9 pm to 10:15 pm EDT (the shadow will linger until 10:45 pm. A reasonable backyard telescope will show the black shadows, but a very good telescope is needed to see the moons, too.

The Great Red Spot takes about three hours to cross Jupiter’s disk. But the planet’s 10-hour rotation period (i.e., its day) means that the spot is only observable from Earth every 2–3 nights. If you’d like to see the GRS, use a medium-sized telescope (or larger). You’ll have your best luck on evenings with steady air — when the stars are not twinkling too much. The best times to try this week are: Tuesday, May 8 at 9:31 pm and Thursday, May 10 at 11:09 pm. All times are given in Eastern Daylight Time, so adjust for your local time zone. Try to look within an hour before or after the times I’ve given.

(Above: This week, Saturn and Mars rise in the middle of the night and remain in view over the southern horizon before dawn, as shown here for 5 am local time.)

At the end of this week, the ringed planet Saturn will start rising in the east just before midnight. You should be able to see its yellow-tinted point of light until about 6 am, when it will sit about 2.5 fist widths above the southern horizon. Reddish Mars will rise about 80 minutes after Saturn, which places it 1.7 fist widths to the lower left of Saturn. Mars is steadily brightening and increasing in size as the Earth’s faster orbit brings us closer to the red planet. We will pass it on the “inside track” in late July.

This week Mercury continues its trek across the eastern pre-dawn horizon. This appearance isn’t very good for observers in mid-northern latitudes. Your best time to hunt for it this week will be between 5:30 and 5:45 am local time. If you live south of the equator, however, Mercury will be very easy to see for the next week or so.

Deep Sky Delights

With the evening sky nice and dark due to the moon’s waning brightness and its late rising, this is a good week to hunt for some of the sky’s dimmer delights — globular clusters. These balls, each composed of up to a million stars, orbit the Milky Way’s centre like bees around the hive. But they are tens of thousands of light-years away from us. And they are easy to see without fancy telescopes!

(Above: The Great Globular Cluster in Hercules, also known as Messier 13, contains perhaps a million densely packed stars. Image by Martin Pugh. NASA APOD for June 14, 2012)

The best globular clusters observable from mid-northern latitudes are now well-placed in the eastern evening sky. In early evening, use binoculars or a backyard telescope to look for 7.7 magnitude Messier 53 in Coma Berenices (Bernice’s Hair) and the slightly brighter cousin, spectacular Messier 3 in Canes Venatici (the Hunting Dogs). These globular clusters are in the sky below the Big Dipper. Messier 3 is a generous fist’s diameter to the upper right of the very bright star Arcturus, and Messier 53 is 1.4 fist diameters to the right of Arcturus.

As the evening wears on, the globulars Messier 5 in Serpens Caput (the Serpent’s Head) and Messier 92 in Hercules will rise high enough to observe. Both of those objects are also around magnitude 6.5. Messier 5 is located a bit more than two fist diameters below Arcturus. Messier 92 is 1.5 fist diameters above, and slightly to the right of, the very bright, white star Vega. For your finale, spend some time enjoying the Great Hercules Cluster, aka Messier 13. It’s on the western (right-hand) edge of the keystone-shaped body of Hercules.

(Above: The eastern evening sky, shown here at 10 pm local time, contains a number of excellent globular clusters from Charles Messier’s famous list of deep sky objects. The handle of the Big Dipper extends downwards from top centre.)

All of these globular clusters will reach their highest altitudes after midnight. In binoculars, they’ll appear as dim fuzzy patches of light. Charles Messier, who cataloged them, thought they resembled comets — his lifelong passion.

Astronomy Skylights for this week (from April 29th, 2018) by Chris Vaughan.

Keep looking up to enjoy the sky! I love getting questions so, if you have any, send me a note.

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