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Computing Easter

19 March 2024

19 March 2024

The Moon and Jupiter (with 4 moons), imaged 2024 March 14 from Alexandria, Virginia
with a Canon EOS Rebel SL2 DSLR and an Omegon Mini Track LX2 mechanical star tracker.

We'll be on leave next week.  Here's what to look for in the next two weeks.

The Moon leaves the bright winter constellations as she courses through the rising spring stars this week.  She waxes to her full phase, which falls on the 25th at 3:00 am Eastern Daylight Time.  Last Quarter occurs on April 1st at 11:15 pm EDT.  Look for Luna about three degrees north of the bright star Regulus in Leo on the evening of the 21st.  On the 26th you will find her just east of Virgo’s brightest star, Spica.  Early risers on the morning of the 30th will find the waning Moon just west of the ruddy star Antares in Scorpius.

The March Full Moon is traditionally known as the Worm Moon, sine the warming soil and spring showers bring earthworms to the surface, much to the delight of birds and fishermen.  Other traditional names are the Crow Moon, Lenten Moon, and Sap Moon.  This year it is also the Paschal Moon, since it is this particular Full Moon that sets the date of Easter for Christians.  Easter is the most important feast day in the liturgical year, setting the dates for the rest of the “moveable feasts” for a given year.  It often coincides with the Jewish observation of Passover, but due to the luni-solar nature of the Hebrew Calendar, this won’t happen in 2024.

Most people think of Easter as the first Sunday following the first full Moon after the vernal equinox, but that is not always the case.  Easter is actually determined by the “Computus”, a formula devised by Dionysius Exiguus in the year 525 CE.  In this scheme, the equinox was defined as occurring on March 21st, and the full Moon was determined by a date in the Hebrew Calendar, 14 Nisan.  By creating tables with the full Moon dates and using the “ecclesiastical” equinox, it was possible for parish priests to compute the date of Easter without assistance from papal authorities in Rome.

The bright Moon washes out most of the spring constellations as March ends.  That said, the early evening still hosts the bright winter constellations, which may be spotted in the western sky as twilight ends.  The first of the bright winter stars to set is Rigel, which dips below the horizon at around 11:30 pm.  One by one, the stars of Orion slip out of sight over the next hour.  By midnight the winter stars are yielding to the rising stars of spring.

The spring sky lacks the dazzle of the winter constellations, with only three fist-magnitude stars scattered among many fainter ones.  The brightest of these is Arcturus, which rises at around 8:30 pm local time.  It is hard to miss since it is the brightest star in the northern sky and the fifth brightest overall.  It has a distinctive rosy tint that seems very appropriate for the season.  The hue tells us that Arcturus is an evolved star that is entering the “shell” phase of its life.  Hydrogen fuel at the star’s core has been exhausted, so it starts fusing in a slowly expanding shell around the core.  This causes the star’s surface to expand and cool, emitting a redder color temperature.  It is the nearest “red giant” star to the solar system, just under 37 light years away.

An easy way to find Arcturus is to locate the Big Dipper asterism high in the northeastern sky.  Follow the “arc” of the stars in the Dipper’s “handle” to Arcturus.  Continuing that line, you can “speed on” to Spica, the lead star of the sprawling constellation of Virgo.  Spica rises about an hour after Arcturus and is a “blue giant” star that gives a nice color contrast to Arcturus.  It lies some 250 light years from us.

By midnight the star Regulus is crossing the meridian in the southern half of the sky.  Its name means “little king”, appropriate for its location as the “heart” of Leo, the Lion.  Look above Regulus for a semicircle of second- and third-magnitude stars that outline the Lion’s head.  His tail is denoted by a right triangle of stars that follow some 16 degrees east of Regulus.

Jupiter continues to slip toward the western horizon with each passing night.  You should be able to spot him about 40 degrees above the skyline as evening twilight falls, and he remains prominent until shortly before he sets at around 11:00 pm.  You’d better hurry if you want to catch a glimpse of him in the telescope.  By the end of the month he’s gone before 10:30 pm. 

The elusive planet Mercury is now putting on his best evening show for the year.  You can find him about 10 degrees above the western horizon about half an hour after sunset.  He will keep pace with the advancing Sun until the 24th, when he reaches greatest elongation.  By early April, he fades as he starts to plunge back toward the advancing Sun.


Commander, Naval Meteorology and Oceanography Command | 1100 Balch Blvd. | Stennis Space Center, Mississippi 39529

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