Skylights: April 2022

April is the month during which the seasonal change in the evening sky is most noticeable. This is due to the Sun’s increasing declination as it moves along the ecliptic, and the resultant later sunsets,  resulting in the stars of the winter sky rapidly descending into the western twilight each passing night.

We start April with Orion, Canis Major, and the constellations comprising the Winter Hexagon shining prominently in the southwestern sky, but by month’s end, only the top half of Orion will be visible above the horizon after twilight, with his three belt stars, Alnitak, Alnilam, and Mintaka, setting due west at a near horizontal orientation.

The Sun enters Aries just after sunset on the 18th. The following morning, April 19, sees the Sun rising before 6am, a mark it will remain through August 20.

April begins with a sighting opportunity for a 17-hour-old Moon, on the 1st. You’ll need an exceptionally clear western horizon to spot the 0.5% illuminated crescent. Begin observing 20 minutes after sunset. The Moon becomes much easier to find the following night, when the 2.9% illuminated crescent lies on the border of Pisces and Aries.

On the 3rd, use the Moon to easily locate Uranus, just 2.5° to its west. As an additional pointer, the 5.8 magnitude star Omicron (37) Arietis lies 42 arcminutes to the east of the magnitude 5.9 planet. This should be a nice sight in binoculars.

On the 4th, the Moon lies within 4° of the Pleiades cluster in Taurus, and on the 5th, it passes 3° from Aldebaran and the Hyades.

First quarter Moon occurs on the 9th, within Gemini, the Twins.

Full Flower Moon occurs on the 16th. This Full Moon is notable for rising near the 1st magnitude star Spica, in Virgo. Moonrise is at 19:33, just 5 minutes after sunset.

The following night, on the 17th, watch the Moon rise, and keep close watch on its eastern limb, as the star Zubenelgenubi (Alpha Librae) reappears from behind the Moon. Because this is a wide double star, we get to see two separate occultations end. The 5th magnitude Alpha 2 appears at 20:56, and the 2nd magnitude primary appears at 21:03. This event is best viewed through a telescope at medium magnification. Both stars appear from the dark limb beyond Mare Crisium.

After midnight on April 19th, the Moon occults another fairly bright star, 2nd magnitude Dschubba (Delta Scorpii). Disappearance behind the bright western limb occurs at 2:17, and reappearance at 3:10.

If you’re up for a bit of an observing challenge, use your telescope to see the waning gibbous Moon pass 8 arcminutes to the north of globular cluster M19 in Ophiuchus at 2:30 on the 20th.

Last quarter Moon occurs on the 23rd, and for the next several days, the waning crescent passes south of the morning planetary parade, with particular interest on the 27th, when it forms a triangle with the two brightest planets, Venus and Jupiter.

The Moon is New on the 30th.

All of the bright planets, except Mercury, appear in the morning sky in April.

Mercury passes superior conjunction on the 2nd, and subsequently enters its best evening apparition of 2022. By the second week of the month, Mercury is already prominently positioned above the western horizon at least a half hour after sunset, and by month’s end, when it reaches greatest elongation, 21° from the Sun, it sets nearly two hours after sunset. Be sure to observe Mercury on the 29th and 30th, as it will be just 1.5° from the Pleiades cluster in Taurus.

Another notable conjunction occurs on the 17th, when Mercury passes 2° north of Uranus. On this night, Uranus is over 18 times farther away from us than Mercury (20.672 AU and 1.124 AU, respectively).

Venus, the brightest object in the morning sky, rises in Capricornus at 4:37 on April 1 and in Pisces on 4:10 on April 30th. 

At the start of April, Venus still appears within a fairly tight grouping with Saturn and Mars, though you’ll notice that the orientation of the planets is changing fairly quickly. Last month, Venus appeared between Saturn and Mars, but in early April, Saturn appears between Venus and Mars, and Saturn and Mars will swap places on the 5th, when the two planets will appear just 22 arcminutes apart.

Additional planetary conjunctions occur on April 12th, when Jupiter appears just 7 arcminutes above Neptune, though the outermost planet will be difficult to observe low in the sky, and through brightening twilight/, on April 27, Venus appears just 27 arcminutes to the west of Neptune; and on April 30, Venus joins Jupiter, and the two brightest planets will be just 1/2° apart.

Jupiter rises at 5:49 on April 1, and by mid-April, it begins to shine through early morning twiligh, although it will be difficult to observe low and through twilight, 

The evening sky in April is an opportune time to explore some of the sky’s lesser-known stars and  constellations.

We begin at the head of Hydra, just below Cancer. Hydra, the water snake, is the largest constellation in the sky, but its long, meandering string of dim stars extending southeastward from its fairly conspicuous head is not very well traveled by northern hemisphere observers. It’s worth getting to know this constellation’s brightest star, Alphard (Alpha Hydrae), which can be found about 16° southeast of the head of Hydra. Despite being as bright as most of the stars in the Big Dipper, Alphard seemingly does not get much attention. Alphard is a class K3 orange giant star, similar to Aldebaran, about 180 light years away. Alphard is about 4.5 times the mass of the Sun, and shines about 950 times as bright.

While Hydra has plenty more to offer, we’ll step off here to explore some other small and dim constellations that don’t hang around in our sky for very long: Pyxis, the Compass, Antlia, the Air Pump, and Crater, the Cup.

Pyxis is one of the smallest constellations, occupying the area of sky roughly between 8.5 and 9.5 hours right ascension, and -20° and -37° declination. 

To find Pyxis, begin at Regulus, draw a line through Alphard, and continue along the line about the same distance. You’ll find yourself in a fairly nondescript region of sky to the east of Canis Major, though not immediately adjacent to it (that would be Puppis, the stern).  Here you’ll find an 8° long line of three stars that points roughly in the direction back to Alphard and Regulus, which should make it fairly easy to find using a binocular sweep. 

The brightest star in the entire constellation is magnitude 3.7 Alpha Pyxidis, which expectedly, is not prominent enough to have an official name designated by the International Astronomical Union. The star is a young class B1.5 blue giant, about 850 light years distant, but otherwise unremarkable other than its position in a constellation seldom visited.

Located at the fringes of the Milky Way, Pyxis does have a few small open star clusters that may be explored with a sizable telescope, including NGC 2627, NGC 2658, and NGC 2818.

Immediately to the east of Pyxis lies Antlia, the Air Pump. Having no resemblance to the object it is meant to depict, Antlia is also populated by a sparse collection of stars no brighter than 4th magnitude. Its constellation figure, as illustrated by the star charts on the IAU’s constellation page, shows an offset chevron connecting the stars Epsilon, Alpha, and Iota. The long, western leg forms a line about 13° long which points towards the northwest corner of Corvus to the east, and the shorter leg, at 9°, points roughly back to Alphard.

In getting to know Antlia, we’ll again focus on the constellation’s alpha star,its brightest, at magnitude 4.3, and also more northerly, so it should be easy to find. Alpha Antliae is at almost the complete opposite end of the spectral classification scale as Alpha Pyxidis. A class K4 giant, Alpha Antliae shines with a luminosity of about 500 suns from a distance of 365 light years.

Antlia contains no notable bright deep sky objects within its borders, but just over its southern border, in the constellation Vela, lies Caldwell 74, a planetary nebula also cataloged as NGC 3132. At roughly the same dimensions and brightness as the more well-known Messier 57 in Lyra, Caldwell 74’s extreme southern declination of -40° will make it a challenge to see.

Continuing northeastward and crossing over the water snake, we reach Crater, the Cup. This small constellation consists of eight 4th and 5th magnitude stars in a figure that actually resembles a  chalice. Sweeping the area of sky to the west of the more prominent Corvus, you should have little difficulty tracing its figure. Crater is one of few constellations noted for “sharing” one of its line segments with an adjacent constellation. In this case, the southwestern edge of its figure is also a part of Hydra. If this area of the sky is somewhat familiar, it may be that you have used the stars of Crater as a starting point on your way to find the fairly well-known planetary nebula NGC 3242 (Caldwell 59), located near Mu Hydrae, just a few degrees to the west.

Most of Crater’s stars, as well as some in neary Hydra and Corvus, are at a similar distance range of about 150 light years. Crater’s brightest star, magnitude 3.6 delta, lacks a proper name, but its alpha star, magnitude 4.1 is named Alkes, meaning the wine cup. 

There are two other officially named stars in Crater, neither of which are naked-eye visible or part of the constellation figure, but are notable as being named by the public in the IAU’s NameExoWorlds campaign. The first is Hunahpú, which is the name given to 8th magnitude star HD 98219 and was chosen by Hondurus. The name represents one of the twin Sun gods from Mayan mythology. The other twin, Ixbalanqué, is the name given to its planet, a super Jupiter orbiting the K0 subgiant star at a distance of 1.2 AU. Hunahpú is 372 light years away.

The other NameExoWorlds star is Amansinaya, or WASP-34. The name derives from the Tagalog mythology of the Philippines and represents the primordial ocean god and protector of fishermen. Its planet, WASP-34b, is given the name Haik, which is the successor of Aman Sinaya in Tagalog mythology. The planet is just over half the mass of Jupiter and orbits so close that it takes only 4.3 days to orbit its parent, a class G5 sunlike star. The system is 430 light years away.

And finally, no adventure to this part of the sky is complete without a visit to the beautiful lenticular galaxy Messier 104, in Virgo.