Skylights: April 2023

We’re now well into spring, and the daylight hours outnumber hours of dark, with each successive day being over two and a half minutes longer than the previous. This becomes quite noticeable by how late evening twilight lasts, and how quickly the stars of the winter sky are dropping out of view in the west.

The Sun begins rising earlier than 6:00am beginning on the 19th, and will not rise after 6:00am until August 22. And while we won’t see 8:00pm sunsets until mid-May, the last extension of astronomical twilight finally fades away after 9:30pm at the end of April. 

The Sun crosses into Aries on the 19th, where it will remain until May 14th.

The 11-day gibbous Moon is 4° north of Regulus at moonset on the 2nd. The Full Pink Moon, in Virgo, rises at 6:54pm on the 5th, and sets at 6:37am on the 6th.

At about 2:00am on the 10th, the 19-day waning gibbous Moon makes a spectacularly close 0.5° pass to the north of Antares (alpha Scorpii). The Moon is last quarter in Sagittarius on the 13th, and on the 16th, the 25-day waning crescent is 4.4° southeast of Saturn. 

New Moon occurs a few minutes after midnight on the 20th, and on the opposite side of Earth from the Eastern US, a hybrid solar eclipse is occurring. While we don’t get to see any part of this eclipse, a year from now, on April 8, 2024, we do get to experience a partial solar eclipse, which is total just to the north and west of us, Seagrave Observatory will experience a deep, 91% partial eclipse.

Just 20 hours after April’s new Moon, a very young Moon sighting opportunity presents itself, with the 0.7% illuminated, 20-hour-old Moon setting at 8:20pm on the 20th. 

The waxing crescent Moon is a few degrees west of Venus on the 22nd, and east of it on the 23rd. On the 24th, it is 2.7° north of open cluster M35 in Gemini, and then passes 2.3° north of Mars on the 25th.

The First Quarter Moon is 3.2° northeast of open cluster M44, in Cancer, on the 27th, and on the 29th, the 9-day waxing gibbous is 4.4° northeast of Regulus, in Leo. Finally, on the 26th, the 10-day Moon is 3.8° southeast of Pollux, in Gemini.

The best evening apparition of Mercury occurs in April, with the planet reaching greatest eastern elongation of 19° on the 11th. 

With Mercury’s position a few degrees north of the ecliptic, the steep angle of the ecliptic relative to the horizon during April evenings keeps Mercury in our sky for well over an hour after sunset for most of the month. This makes it inviting to view its rapidly changing phase in a telescope. During the first week of April, Mercury presents a small gibbous phase. On the 8th, its globe is 50% illuminated and 7 arcseconds across, and by the 13th, it narrows to a 33% illuminated crescent over 8 arcseconds across. On the 21st, when it is just 0.66 AU from Earth, it grows to over 10 arcseconds and is 10% illuminated. And while you’re observing Mercury on the 21st, look 3.8° to its southeast, to find Uranus. 

Venus’s prominent position over the western horizon for more than three hours after sunset during April gives clear illustration to its moniker as “Evening Star.” And while its brilliant magnitude -4.0 appearance high in our spring sky may seem to be the brightest a planet can appear, it is only half as bright as it will be in September, when it peaks at magnitude -4.8.

Venus closes to within 1 AU of Earth on the 27th, when its 67% illuminated gibbous disk spans 16.7 arcseconds, slightly larger than the globe of Saturn. 

As Venus tracks across the sky rather quickly, compared to other planets, notice its changing position with respect to the Pleiades in Taurus. Venus spends the second week of April within the same binocular field of view of the sky’s most prominent open cluster, passing just 2.4° south of it on the 10th. It then “enters” the Winter Hexagon by crossing the line connecting Aldebaran and Capella on the 22nd, and on the 25th it passes within 1° north of open cluster NGC 1746.

There are few things that produce a more stunning sight in the deepening twilight of the evening sky than an Earthshone crescent Moon with sparkling Venus nearby. One of these opportunities presents itself on the 22nd, with the 9% illuminated 1.9-day Moon lying directly between Venus and the Pleiades. 

You may notice that throughout April, Venus is moving across Taurus, territory that Mars was passing through not too long ago. Since we’re now on the opposite side of the Sun as Mars, it appears to be moving fairly quickly as well, in the next constellation to the east, Gemini.

As April begins, the Red Planet is just 1.5° from open cluster M35. On the 14th, it forms a twin with 3.0 magnitude star Mebsuta (epsilon Geminorum) just 0.2° to its northwest. On the 25th, the 5.6-day crescent Moon lies 2.3° to its north.

While Mars still shines at a respectable first magnitude, it is now over 1.5 AU away, and its diminutive 6 arcsecond disk will show detail only through large aperture telescopes under ideal seeing conditions. 

April 2023 is not a good time to observe Jupiter, as the giant planet is in conjunction on the 11th, and is only above the horizon for a few minutes after sunset at the beginning of the month, and a few minutes before sunrise at the end of the month.

While it has been hovering low in the southeast before sunrise for the past several weeks, Saturn begins to rise early enough in April to be easily observed.

On the 16th, the 25.5-day crescent Moon lies 4.4° southeast of Saturn. While we weren’t looking (when it was behind the Sun two months back), Saturn moved into Aquarius and now rises at about the same time as Algenib (gamma Pegasi), 36° to its left, so when you see the Great Square asterism rising, you’ll know Saturn is coming up soon.

Uranus, in Aries, is visible early in the month a few degrees east-northeast of Venus, but quickly falls into the twilight as the month progresses. Find it 4.0° southeast of Mercury on the 20th, then expect it to be hidden from view for the next several weeks.

Neptune, now in Pisces, is just far enough from the Sun to be seen low in the east before twilight at the end of the month. At magnitude 8.2, it will require a telescope to see its distant dim glow 4.6° south-southeast of lambda Piscium.

Dwarf planet Ceres, which reached opposition on March 20th, remains easily visible in Coma Berenices throughout April. As its distance from Earth steadily increases from 1.6 to 1.8 AU, it dims slightly, from magnitude 7.0 to 7.6, well within range of small telescopes, even when the bright Moon is nearby early in the month. At the beginning of April, Ceres is located in the vicinity of Messier galaxies M98, M99 and M100. By the end of April, it is just 2.5° northeast of Denebola (beta Leonis).

While Ceres is our closest, and easiest to observe dwarf planet, April presents observers with the best opportunity of the year to observe two much more distant, and challenging, objects. These are Haumea and Makemake. Discovered in 2004 and 2005, respectively, each of these objects is about ⅔ the size of Pluto, and both also have moons.

Haumea, at a distance of 49.28 AU, reaches opposition on April 30. Located about ⅔ of the way from Arcturus to zeta Bootis, the distant world shines at magnitude 17.5. While this may be within visual reach of very large telescopes (think 30 inches or more), it is relatively accessible to observers with telescopes of significantly smaller aperture that are equipped with a tracking mount and camera.

Makemake is slightly more distant, at 51.8 AU, and although it is approximately the same size as Haumea (though not the same shape, Haumea is an elongated spheroid), it has a somewhat higher albedo, and shines about a half magnitude brighter than Haumea. Makemake reaches opposition on April 12, and is located 1° east of the midway point on a line connecting Diadem (alpha Coma Berenices) and beta CrB.

It is notable that Pluto, perhaps the most well-known dwarf planet, has crossed into another constellation. Now a resident of Capricornus, the last time that Pluto crossed a constellation boundary was in 2006, when it crossed from Serpens into Sagittarius. Pluto is still rather low in the southeast before dawn.

Asteroid 2 Pallas dims from 8th to 9th magnitude through April, as its distance from Earth increases to beyond 2 AU by the 23rd. Still, it should be easy to track with a small telescope as it makes its way northeastward from Monoceros into Canis Minor. At the beginning of April, it is a magnitude 8.3 speck about halfway between Sirius, in Canis Major, and Procyon, in Canis Minor. As the month progresses, it moves closer to Procyon, becoming only 2.8° south-southeast of the magnitude 0.4 star on the 25th.

Looking beyond the solar system, this is the time of year when we can explore our extragalactic surroundings. In early April, when our side of Earth points directly away from the Sun at midnight, we are facing straight out of the Milky Way galaxy. The North Galactic Pole lies nearly overhead in the middle of the night. If we could see the galactic pole, it would lie about midway between beta and gamma Comae Berenices. If we had a truly dark sky, horizons devoid of any obstructions, and air significantly more transparent than average, say, if we were at the peak of a high mountain, we would see the Milky Way nearly encircling our horizon. It would be a distinct band of light extending from below Procyon in the west, through the Winter Hexagon, past Capella, northward through Perseus, Cassiopeia, Lacerta, and Cetus, then through the Summer Triangle down the long axis of Cygnus in the east, southward through Aquila and Scutum, and finally dropping out of view below Antares, in the southeast. In reality, from our latitude of about 42°, there is about one quarter of the Milky Way that we never see.

Since we’re looking at the Milky Way from within, and it is a relatively flat disk, gazing out along its axis affords us the ability to see deep into space, well beyond the occluding stars, dust and gas of our own galaxy. This is the reason that this region of sky was chosen for the original Hubble Deep Field, a sample survey of previously uncataloged distant galaxies conducted with the Hubble Space Telescope, using over 100 hours of exposure time in 1995. The Hubble Deep Field is in a 2.7 arcminute wide patch of sky located just north of the Big Dipper, at approximately the intersection of a line drawn north from Phecda through Megrez (gamma and delta UMa), and from Alkaid through Mizar (eta and zeta UMa).

While we cannot see any of the 3,000 galaxies revealed by the Hubble Deep Field image, some of which are 12 billion light years distant, we can find many of the showpiece galaxies from Messier’s catalog in this region of sky. The one closest to the Milky Way’s north galactic pole is M64, in Coma Berenices. M64 is a relatively bright spiral galaxy that is fairly easy to find. Coma Berenices contains a large open cluster, known as Melotte 111 (also Coma Star Cluster), that is visible to the unaided eye from a dark site, and is otherwise a delight to explore with binoculars. On a dark night with no moonlight, sweep one binocular field east from the southeastern extent of the cluster to locate M64’s 8.5 magnitude glow. Another way to locate it is to move ⅓ of the way along a line connecting Diadem (alpha) and gamma Comae Berenices, at the northern point of Melotte 111. Turning a telescope towards M64 and applying medium power reveals the interesting structure that gives it its nickname–the Black Eye Galaxy. Its bright nucleus is surrounded by a thick ring of dust that occludes much of the light from the galaxy’s core, but is surrounded by relatively bright spiral arms just outside of its dust ring.

As you gaze out at M64, high overhead during the late hours of these spring evenings, think back to the galaxies in the Hubble Deep Field, which are 700 times more distant. And the Coma Star Cluster? Many of its members are about 275 light years away, making it one of the closest star clusters to our solar system. “Nearby” M64 is 61,000 times more distant.