Sky This Week is brought to you in part by Celestron.
Friday, May 22
Uranus is in conjunction with the Sun at 10 A.M. EDT, so it is not currently visible in our sky. It will return in June, when it will appear in the predawn hours.
The famous Summer Triangle asterism, bounded by the bright stars Vega in Lyra, Deneb in Cygnus, and Altair in Aquila, is still waiting a few hours after sunset to clear the horizon. But there’s another triangular asterism high in the sky after sunset these days, aptly named the Spring Triangle.
This cosmic shape is marked at its three points by magnitude 1.0 Spica in Virgo, magnitude 2.1 Denebola in Leo, and magnitude –0.1 Arcturus in Boötes. An hour after sunset, these bright stars have popped into view in the south. Spica is the lowest, some 35° high; to its upper right is Denebola, and to Denebola’s upper left is Arcturus.
The Moon is also nearby, just 3° from Leo’s brightest star, magnitude 1.4 Regulus. If Denebola marks the tip of the Lion’s tail, Regulus is situated where the great cat’s heart would be located. Keep an eye on the Moon as the region sets; it is approaching Regulus and will pass some 5’ south of the star early tomorrow morning. Some parts of Oceania will see the Moon pass in front of the star in an occultation, but this event is not visible from the U.S.
Sunrise: 5:39 A.M.
Sunset: 8:15 P.M.
Moonrise: 11:46 A.M.
Moonset: 1:17 A.M.
Moon Phase: Waxing crescent (46%)
Times for sunrise, sunset, moonrise, and moonset are given in local time from 40° N 90° W. The Moon’s illumination is given at 10 P.M. local time from the same location.
Saturday, May 23
The Moon passes 0.08° south of Regulus at 3 A.M. EDT. A few hours later, First Quarter Moon occurs at 7:11 A.M. EDT.
First Quarter Moon offers some great targets for lunar observers. Plus, the great thing about the Moon is that it’s easy to find and focus on, so it doesn’t matter whether you’re a new or experienced skywatcher. Tonight, the Moon is still high in the southwest an hour after sunset, with the entire eastern half of the nearside now illuminated.
You don’t need to wait for full dark to zoom in on the Moon with a telescope. Concentrate your view along the terminator, the dark line dividing lunar night and day. In the north, just above the broad, dark oval of Mare Serenitatis, you’ll see two circular craters in a north-south line: these are Aristoteles in the north, and Eudoxus just south of it. Now scan just west of Aristoteles. Not far away is a large, dark slash in the brighter, rugged terrain. This is Vallis Alpes, a valley cutting through the Montes Alpes lunar mountain range on the northeastern rim of Mare Imbrium. This gash in the mountains is roughly 103 miles (166 kilometers) long and some 10 miles (16 km) wide at its widest. Note that one side of the valley — the southern side — appears much straighter than the slightly rounded northern edge. There is a thin, hard-to-spot rille, or groove, running lengthwise through the center of the valley. It can be a challenging detail to view, but crank up the magnification and see if you can spot it. If not, come back in a day or two, when there is more illumination in this region, and try then.
Sunrise: 5:38 A.M.
Sunset: 8:16 P.M.
Moonrise: 12:55 P.M.
Moonset: 1:45 A.M.
Moon Phase: Waxing gibbous (57%)
Sunday, May 24
Moving through vast Virgo, 11th-magnitude asteroid 13 Egeria is passing near some stationary background stars tonight, offering an excellent chance to chart its motion over the course of a few hours.
You can begin looking for the main-belt world as soon as it’s nice and dark, as Virgo is already high in the sky. Two hours after sunset, Virgo’s brightest star, Spica, stands nearly 40° high in the south. From this bright star, slide your telescope slowly upward (north) 5°. Now you’re in Egeria’s realm.
Check the chart above for the positions of stars down to roughly Egeria’s magnitude. (Note that you may see even more stars in your scope, if you can pick up fainter ones.) Egeria is near a smattering of 9th- and 10th-magnitude suns; early in the evening, it’s roughly 10’ northwest of a magnitude 8.2 star and about 25’ southeast of a magnitude 8.5 star. There’s also a 10th-magnitude star about 6’ to Egeria’s southeast.
Take a photo or sketch out the pattern you see, and then take some time to check out the other sky wonders on your list. Come back in a few hours — say, three — and snap another photo or make another sketch. Has one of the dots here moved just slightly? That’s Egeria, sliding slowly to the southwest relative to the stationary background stars.
Sunrise: 5:38 A.M.
Sunset: 8:17 P.M.
Moonrise: 2:01 P.M.
Moonset: 2:09 A.M.
Moon Phase: Waxing gibbous (67%)
Monday, May 25
After reaching superior conjunction midmonth, Mercury is increasing in visibility and should be easily identifiable now in the evening sky. Just half an hour after sunset, step outside to see if you can spot the solar system’s smallest world, now shining at a respectable magnitude –1.1. It should be 6° above the western horizon.
You will, of course, also spot brighter Venus and Jupiter. Jupiter is highest in the sky, now in eastern Gemini and shining at magnitude –1.9. Venus, a little lower, sits closer to the Twins’ feet and lies between Jupiter and Mercury, shining at magnitude –3.9. If you have trouble spotting Mercury, simply draw a line from Jupiter down through Venus and continue it toward the horizon. Mercury lies along that line.
Through a telescope, compare the sizes and illuminated extents of Venus and Mercury. The former now appears some 13” across, with 81 percent of its visible disk (cloud tops) lit. Mercury is a slightly more illuminated at 84 percent lit, but it’s less than half the size of Venus, stretching only 6” wide in our sky.
Sunrise: 5:37 A.M.
Sunset: 8:17 P.M.
Moonrise: 3:05 P.M.
Moonset: 2:30 A.M.
Moon Phase: Waxing gibbous (76%)
Tuesday, May 26
Tonight, let’s compare the appearance of the Moon to that of a week ago to see the effects of lunar libration, the “nodding” motion caused by the tilt of the Moon’s orbit around Earth.
High in the south shortly after sunset, the waxing Moon is in Virgo, not far from the bright star Spica, which has starred in several entries this week. With much of its face now illuminated, look in the center of the lunar northern hemisphere for Mare Serenitatis. North and just slightly east of this ancient lava plain is a crater normally situated much closer to the limb: Endymion. Shallow and flat-bottomed, Endymion typically appears oval-shaped due to foreshortening, but tonight it should look more circular, as it appears farther from the limb thanks to libration. Stretching some 76 miles (122 km) across, Endymion is some 3.9 billion years old.
Compare the images below, showing the location of Endymion tonight and a week ago, on the 19th. Together, they illustrate the magnitude of change libration can bring.
Sunrise: 5:37 A.M.
Sunset: 8:18 P.M.
Moonrise: 4:08 P.M.
Moonset: 2:51 A.M.
Moon Phase: Waxing gibbous (84%)
Wednesday, May 27
Ready for a ruby-red target? Let’s visit the star Y Canum Venaticorum, also famously known as La Superba, “the superb one.”
As soon as it’s dark, look for the constellation Canes Venatici high overhead. Don’t know where it is? It’s quite easy to locate: You’ll find its two bright stars nestled just beneath the curve of the Big Dipper’s handle. The star we’re after, La Superba, is located some 10.6° south-southwest of 2nd-magnitude Alioth (Epsilon [ε] Ursa Majoris), the third star from the end of the Big Dipper’s handle. Alternatively, you can find La Superba just over 7° north-northwest of Cor Caroli, Canes Venatici’s 3rd-magnitude alpha star.
La Superba is a variable star, meaning it glows anywhere from magnitude 4.8 to 6.3. Currently it is near that minimum, with recent reports from the American Association of Variable Star Observers placing the star around 6th magnitude. Binoculars or a small telescope should show it easily, and you’ll know it by its deep red color. La Superba shines with this hue because it is a carbon star, whose atmosphere (which is abundant in carbon) absorbs blue wavelengths while letting the red shine through, so the latter is what reaches our eyes.
Sunrise: 5:36 A.M.
Sunset: 8:19 P.M.
Moonrise: 5:10 P.M.
Moonset: 3:13 A.M.
Moon Phase: Waxing gibbous (91%)
Thursday, May 28
Today’s target sits near the head and claws of Scorpius the Scorpion, highest in the south shortly after midnight. M80 is a lovely, tightly-packed globular cluster of ancient stars located about 4.5° northwest of Antares, the heart of the Scorpion.
Visible in binoculars or any small scope, M80 glows at magnitude 7.3. It spans just 10’, making it quite compact. Although small instruments will easily pick it up, you’ll see the cluster through them as a bright, small fuzzy ball. If you want to begin resolving M80’s stars, try for a bigger scope and bump up the magnification. Many observers note that because it is so compact, the cluster looks a bit like a cometary nucleus without a tail.
M80 sits some 32,600 light-years away and contains hundreds of thousands of stars packed into a region of space about 95 light-years in diameter.
Sunrise: 5:35 A.M.
Sunset: 8:20 P.M.
Moonrise: 6:13 P.M.
Moonset: 3:37 A.M.
Moon Phase: Waxing gibbous (95%)
Friday, May 29
High in the northeast this evening, located within the head of Draco the Dragon, is one of the sky’s most famous and easy-to-split double stars. Nu (ν) Draconis appears to shine as a single 4th-magnitude star to the unaided eye. In truth, it comprises two 5th-magnitude components spaced so widely apart (62”, or a little over an arcminute) that you can spot both in binoculars or even your telescope’s finder scope. They appear nearly identical, each glowing with a soft white light. Both are roughly nine times as bright as the Sun and about 1.7 times as massive as our star. They circle each other every 44,000 years.
Nu1 is the western component, while Nu2 is the eastern star of the pair. According to the late stellar expert Jim Kaler, perhaps the largest difference between them is that Nu2 is itself a double, with a much smaller secondary that orbits the primary in less than 40 days. However, perhaps they aren’t so different after all: Kaler notes that astronomers suspect Nu1 may also be a double, but this has not yet been confirmed.
Sunrise: 5:35 A.M.
Sunset: 8:21 P.M.
Moonrise: 7:17 P.M.
Moonset: 4:04 A.M.
Moon Phase: Waxing gibbous (98%)
