Skylights: March 2024

The March night sky brings about some of the most notable changes that can occur during any given month; some are welcome, and some of them can be challenging.

Most notably on the challenging side, the Sun moving into the northern celestial hemisphere through the equinox gives us fewer hours of darkness to observe, and combining this with the annual “spring ahead” clock change, our evening hours are shifted even further into the daylight. For evening observers, that means that the end of astronomical twilight, when the Sun is 18° below the horizon and the sky is fully dark, changes from 7:09pm EST at the beginning of the month, and moves to 8:46pm EDT at month’s end, giving just over three hours of darkness to observe before midnight.

Among the welcome changes are that the cold, blustery nights are mostly behind us. And with the warmer nights, by the end of March, the spring peepers will be making their annual return, their nightly chirps being the harbinger of a chorus of nocturnal wildlife that will accompany our observing from now until the cool nights of autumn.

March also brings the opportunity to observe the entirety of the Messier catalog in a single night.

But first, a tour around our solar system.

The Sun enters Pisces on the 11th, after having spent the past 40 days moving through Aquarius.

For the past six months, the Sun has been positioned below the celestial equator, that is, in the southerly declinations. At 11:06pm EDT on the 19th, it crosses the equator and moves into the northern sky, shining directly onto Earth’s northern hemisphere for the first time since the end of last summer. This is the equinox, which translates to “equal night,” although due to the Sun having a finite apparent diameter and atmospheric refraction, the true “equal night” occurs on the 17th for our latitude.

Additionally, although unrelated to the position of the Sun, it does affect the times of sunrise, sunset, and other events going forward. Our semiannual daylight time clock shift occurs at 2:00am on the 10th, shifting local time ahead an hour to Eastern Daylight Time, which is four hours behind UTC.

The last quarter Moon passes a scant 0.1° south of Antares (alpha Scorpii) on the morning of the 3rd. For observers in the southeastern United States, the Moon will occult the star. The two objects are closest a few minutes before 3:00am EDT. This is a very good demonstration of the Moon’s motion in the sky relative to the background stars. Begin by tracking Antares with a telescope at medium magnification, and use the Moon’s terminator (the division between the light and dark halves of the Moon) as your reference line. The eastward movement of the Moon should be apparent in under one minute. 

The Moon is new at 4:00am EDT on the 10th, beginning Lunation 1252. Just 15 hours later is an opportunity to observe the very young, 0.5% illuminated crescent Moon less than 30 minutes after sunset. It will be almost directly below Mercury by 3.3°.

After joining Jupiter, 2.8° to the planet’s north on the 13th, the Moon visits the Pleiades once again on the 14th, when the 4-day crescent passes just 1.0° southwest of the cluster. The closest appearance takes place just before Moonset, around 11:00pm EDT.

On the 16th, the Moon passes 1.5° east-southeast of Elnath (beta Tauri) in the early evening. Only 5 hours later, just after midnight on the 17th, the Moon is first quarter.

Through its waxing gibbous phase, the Moon is 2.0° south-southwest of Pollux, in Gemini, on the 19th, 3.0° north-northwest of the Beehive Cluster, M44 in Cancer, on the 20th, and 2.2° north-northeast of Regulus, in Leo, on the 22nd.

The Full Worm Moon occurs on the night of the 24th-25th, and, as this full Moon is only a half-cycle from April’s total solar eclipse, the Moon undergoes a lunar eclipse. Unfortunately, however, this is only a penumbral eclipse, which means that no part of the lunar disk slips into the dramatically dark umbral shadow, the shadow under which no part of the Sun illuminates any section of the Moon. In a penumbral eclipse, an observer on the surface of the Moon covered by the penumbral shadow would witness a partial eclipse of the Sun, with Earth covering a portion of, but not the entirety of, the Sun’s disk.

The penumbral eclipse begins at 12:53am, peaks at 3:12am, and ends at 5:32am EDT on the 25th. At maximum eclipse, the Moon lies almost entirely within the penumbral shadow, with just a small position of the Moon’s upper limb not covered. While penumbral eclipses are not as conspicuous as a partial umbral eclipse, you may be able to perceive a distinct gradient that runs darker towards the southern limb of the Moon.

During its waning gibbous phase, the Moon passes 2.5° east of Spica, in Virgo, on the 26th, and on the 30th it makes another pass by Antares, this time at 3.0° to the west.

Mercury moved past superior conjunction on February 28, and appears in the evening sky during March for its most favorable evening appearance of 2024.

The first week of the month has Mercury setting less than an hour after the Sun, but each night it moves northeastward, away from the Sun, in a line perpendicular to the horizon, through greatest elongation on the 24th, when it will be 18.7° from the Sun, and setting over 90 minutes after sunset. Mercury’s latest setting during this apparition is 8:42pm EDT on the 26th.

Given Mercury’s position, relatively high above the horizon during evenings, makes this the best time to observe the tiny planet’s rapidly changing phases. As Mercury climbs higher each night, its illuminated fraction decreases as its apparent size increases. It goes from 91%, 5.4 arcsecond gibbous on the 10th, to 7.1 arcsecond 50% “half-moon” on the 22nd, and ending the month at a 9% crescent growing to over 9 arcseconds.

If you’re doing your Messier Marathon at the end of the month, you will find the galaxy M74 in Pisces just 2.5° to the north of Mercury.

Venus continues to recede into the predawn twilight during March. Although it rises less than an hour before sunrise, its magnitude -3.9 brilliance maintains its ease of visibility for observers with an unobstructed east-southeast horizon.

As Venus is on its way to superior conjunction in June, and is over 1.5 au away, its gibbous phase is now nearly indiscernible on its nearly 11 arcsecond globe.

The waning crescent Moon is 5.7° south-southwest of Venus on the 8th.

Have you seen Mars lately? Chances are you last observed the Red Planet six months ago, when it was still low in the western sky after sunset. Although Mars has been a morning planet since it passed conjunction back in November, it has remained too close to the Sun to be easily observed.

While Mars doesn’t rise into astronomical darkness during March, it is above the horizon at least an hour before the Sun. It is about as bright as Pollux, and should be relatively easy to see, at least with binoculars, low on the east-southeast horizon.

At the beginning of March, it is within 4° west of Venus, and on the 8th, the waning crescent Moon is 4.8° to the southeast, appearing almost directly below Mars.

At over 2 AU distant, Mars appears as a tiny, 4.5 arcsecond globe in a telescope.

Jupiter, which has maintained a prominent position high in the south for the past few months, is now slowly receding into the southwest in the evenings.

The 3-day crescent Moon is 2.8° north of Jupiter in the 13th, presenting an opportunity to view not just two, but six solar system bodies at the same time in a small telescope. Use your lowest magnification to try to fit both Jupiter and the Moon in view. Around Jupiter, find its Galilean moons Europa to the planet’s east, and Io, Ganymede and Callisto to its west. You may notice what appears to be an extra moon to the north of Jupiter; this is not a moon, but omicron Arietis, a class B9 main sequence star that lies over 590 light years away. 

By the end of March, Jupiter sets by 10:00pm EDT.

Saturn, which passed conjunction in late February, is low in the morning sky in March and difficult to observe as it will be lost in twilight. The first opportunity to locate it occurs on the 22nd, when brilliant Venus, 0.6° to its northeast, points the way.

Although the position of Uranus in southern Aries hasn’t changed significantly all year, you may notice that Jupiter is slowly getting closer to it. This is indicative of Jupiter’s higher orbital velocity, combined with its nearer distance to Earth, resulting in a higher parallax. The apparent angular distance between the two planets closes from 8.1° on March 1 to just 3.4° on March 31.

Besides using Jupiter as a guide, you can also find Uranus about 2.5° south-southwest of the magnitude 4.4 star Botein (delta Arietis).

Neptune is in conjunction on the 17th, and won’t be visible until late April.

The third-discovered asteroid, Juno, reaches opposition on the 3rd in Leo. At 1.68 au distant, the 250-kilometer object shines at magnitude 8.5 in early March. On the 1st, it lies just 0.6° west-northwest of 58 Leonis, a magnitude 5.8 star that is 15° southeast of Regulus. From there, it moves northwestward towards rho Leo, coming within 1.3° to the southeast of the magnitude 3.8 star at the end of the month.

Dwarf planet Ceres is moving higher in the morning sky, making it easier to observe. It lies within 1° of the large and bright globular cluster M22 in Sagittarius during the first week of the month, and any size telescope will be capable of picking up its magnitude 8.9 stellar speck, which is still just over 3 au away from Earth.

Pluto makes its reappearance in the March sky, but only low in the southeast during the latter half of the month. It will not reach optimal viewing position for a few more weeks, but if you don’t want to wait, you can find the distant, magnitude 14.6 dwarf planet 0.9° south of the magnitude 5.3 star 4 Capricorni.

4 Vesta, our brightest asteroid, fades to 8th magnitude as it continues its easterly trek through eastern Taurus, and remains visible in binoculars high in the sky.

On the 6th, it lies on the line bisecting Elnath and Tianguan (beta and zeta Tauri, respectively), and is 1.6° north of M1, the Crab Nebula. By month’s end, Vesta lies just 2.1° west of open cluster M35 in Gemini.

2 Pallas, though not as bright at magnitude 9.3, is in a good position for observing in the morning sky, as it is located in southern Hercules, In early March, the 510-kilometer asteroid is about 6° west of kappa Ophiuchi, and from there it arcs northward, towards the midpoint of a line connecting Rasalgethi (alpha Herculis) and gamma Herculis, where it lies at the end of March.

There are a few comets in the March sky worth tracking.

Comet 12P/Pons-Brooks is the comet to watch in the evening sky. Visible in the west at the end of twilight, in early March it is moving eastward through Andromeda, just north of the Great Square, by a distance of about half the height of the square. With the bright Moonlight out of the way, its 8th magnitude glow should be visible in binoculars. The comet is expected to brighten over the next few weeks, as it quickly moves roughly parallel to the horizon, finally meeting up with Hamal (alpha Arietis), which it will be within 0.5° of on the 30th and 31st.

Comet 62P/Tsuchinshan 1 has peaked around magnitude 8 and can be found slowly moving westward through Virgo, a few degrees south of the Realm of the Galaxies. This should be a relatively easy pick-up while conducting the Messier Marathon. 

Comet 144P/Kushida, although a bit dimmer at magnitude 9, but is notable for being positioned high in the south in the early evening. On the 1st, it is just 4.0° south of Tianguan (zeta Tauri), and within the same 1° field of view of 119 Tauri, a 4th magnitude M2-class (red) supergiant star that should make for a nice contrast with the green-tinted comet when viewed in larger telescopes.

The comet then sails eastward through the club of Orion, and passes within 1.5° north of Alhena (gamma Geminorum) on the 21st.

For our latitude of around 42° north, March 16-20 is usually the ideal window during which to conduct the annual Messier Marathon, that is, a single night observing session, during which it is possible to see all 110 objects in the Messier catalog of deep sky objects. However, this year, the first quarter Moon lands on the 16th, and doesn’t depart the sky until a few minutes before 3:00am. As the extra ambient light produced by the Moon interferes with observing some of the fainter fuzzies, it would be optimal to try the marathon a little earlier this year.

On the night of the 14th-15th, the 4.8-day, 28% illuminated crescent Moon departs the sky a few minutes past midnight, making the galaxy-hunting portion of the effort unimpeded by the Moon’s ambient skyglow.