An ‘X’ marks the First Quarter Moon, Dawn Venus Stretches West, and Jupiter gets Spotty!

Star Walk
6 min readMay 29, 2017

Lunar X on Thursday Evening

A few times a year, for a few hours around the First Quarter Moon, a feature called the Lunar X becomes visible in strong binoculars and small telescopes. When the rims of the craters Parbach, la Caille, and Blanchinus are illuminated from a particular angle of sunlight, they form a small, but very clear and bright X-shape. It’s located on the terminator about one third of the way up from the southern pole (bottom) of the Moon (at lunar coordinates 2° East, 24° South). The prominent round crater Werner sits to the lower right. I’ll post a photo here.

The next Lunar X will form in daylight on Thursday evening about 6 pm Eastern Time, peak around 7 pm, and last until at least 8 pm (as the sun is setting). This is a global event. In some parts of the world, the moon will be in a dark sky during that period, making the sighting easier. For the correct time, adjust for your difference from the Eastern Time zone. Let me know if you see it!

The Moon and Planets

This week brings us the First Quarter Moon, which occurs once per monthly orbit when it is sitting at a 90° angle between the Earth and Sun. The moon reaches that position Thursday morning, so it will appear half illuminated on both Wednesday night and Thursday night. During the days around first quarter, the moon is a spectacular sight in binoculars and telescopes, because the sun’s light is striking the hemisphere we see at a shallow angle. All along the terminator boundary that separates the lit and dark sides, long black shadows are cast by the brilliantly lit crater rims and mountain peaks. Every hour (and every evening), the terminator shifts to the moon’s west (our east), highlighting new vistas. As a bonus, the first quarter moon is nicely positioned in the western evening sky.

Moon is Solar Walk 2 app

Starting tonight, you can watch the moon wax from a thin crescent to a gibbous (more than half illuminated) disk next Sunday. Meanwhile, on Monday night, it lands only a 3° (a few finger widths) to the lower left of the Beehive star cluster in Cancer (the Crab). Use binoculars to spot the large loose group of stars swarming near the moon. Finally, on Saturday evening, June 3, the waxing gibbous moon will appear less than 2° (two finger widths) above Jupiter. The moon and bright planet will be in the southern sky at sunset and then set together in the west before dawn, making a pretty sight for unaided eyes and binoculars all evening, and a fine photo opportunity. Virgo’s brightest star Spica will be situated about ten degrees to the lower left of the pairing.

Elusive Mercury continues its tricky morning apparition this week. The best time to see the little planet will be around 5:10 am local time, soon after it has cleared the eastern horizon. By about 5:20 am, the brightening sky will be overwhelming the elusive planet. Look for Mercury in the same location all week — low in the west about two outstretched fist diameters to the south of where the sun will rise. You’ll need a very low horizon free of trees, etc. because, at 5:10 am, the planet is only a few finger widths above the true horizon.

Mercury’s trajectory in Star Walk 2 app

Extremely bright Venus continues to shine in the eastern pre-dawn sky after it rises at 3:30 am local time. On Saturday, June 3rd, it will reach Greatest Western Elongation, its widest angle west of the Sun. Viewed in a telescope around now, the planet will exhibit a half-illuminated phase. Starting next week, it will begin a slow swing back towards the Sun, but it will get better and better for viewing because the morning Ecliptic is rapidly tilting higher as we approach the first day of summer, and Venus (and Mercury) are carried higher at the same time.

On Friday and Saturday, Venus’ motion eastwards (left) will carry it about 1.5° to the lower right of the dim blue-green planet Uranus. Under a dark sky, say just before 4:30 am local time, Uranus can be seen in binoculars or a telescope. If you’re already out with the telescope, try a peek at Neptune, which rises about 2 am local time. For the next month, it’s about 2.5° to the lower left of the star Hydor in Aquarius (the Water-Bearer), which is low in the southeastern pre-dawn sky.

Venus’ trajectory in Star Walk 2 app

Yellowish Saturn rises about 10 pm local time this week, and remains visible until it’s hidden by the brightening dawn around 5 am, when it’s 1.5 fist diameters above the southwestern horizon. Within two weeks, Saturn will be rising at sunset, and be brightest and closest for the year.

Jupiter is the brightest object in the evening sky this week. As darkness sets in, it is halfway up the southern sky, and it sets in the west during the wee hours of the night, local time. Jupiter’s four large Galilean moons are easily visible in a small telescope. A larger telescope will also show the Great Red Spot and the round black shadows cast by Jupiter’s moons when they cross (or transit) the planet. Here are the best events in Eastern Daylight Savings Time. (Simply add or subtract the appropriate hours to convert them to your time zone.)

Jupiter’s trajectory in Star Walk 2 app

Another great Jupiter double shadow transit event occurs on Saturday evening, June 3. Starting at 10:21 pm EDT, Europa’s black shadow will join Io’s already in transit across Jupiter. The two shadows cross at different latitudes, with Europa’s much closer to the planet’s north pole. Both shadows will move off the planet about 12: 20 am.

The Great Red Spot is visible on Jupiter for about three hours centred on Tuesday, May 30 at 1:50 am, and Tuesday evening at 9:41 pm (in twilight), Thursday evening June 1 at 11:20 pm, and Sunday, June 4 at 12:59 am.

Binocular Comet Update

The young moon this week aids in looking for comets. I posted finder charts for the paths of several visible ones during May here. In binoculars and low power telescopes, expect the comets to appear as faint greenish blobs (quite different from a star). If a comet develops a tail, it will point roughly away from the Sun. If you find one in a telescope, watch for 15 minutes or so — you’ll see it moving with respect to the stars nearby.

Comet 41P/Tuttle-Giacobini-Kresak is visible all night, but it’s highest in the sky before dawn. This week, look in the eastern evening sky about as high as the very bright star Deneb and just to the right of the line connecting Altair and Vega. It’s dropping lower and westward, roughly towards Saturn.

Comet 41P/Tuttle-Giacobini-Kresak in Star Walk 2 app

Comet C/2015 V2 (Johnson) is an all-night binocular comet that is still brightening — visible in the southeastern sky as soon as it’s fully dark. It is still moving south through the constellation of Bootes (the Herdsman) in a direction towards that constellation’s brightest star, Arcturus. — ending this week about a palm’s width to the upper left of the star.

Stargazing News for this week (from May 28th) by Chris Vaughan.



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