Chasing Orion

The "light dome" of Washington, as seen from Shenandoah National Park
Imaged from Thorofare Mountain Overlook, Mile 40.5, Skyline Drive,
30-second exposure @ f/4, ISO 3200, made with a Canon EOS Rebel SL2 DSLR

The Moon wanes through her crescent phases in the pre-dawn sky this week. She skirts the southern horizon as she passes through the rising stars of summer, crossing the “Teapot” asterism of Sagittarius on the mornings of the 15th and 16th. New Moon occurs on the 21st at 1:23 pm Eastern Daylight Time.

The vernal equinox occurs at 5:24 pm EDT on the 20th. At this time, the center of the Sun’s disc reaches an ecliptic longitude of zero degrees and signals the beginning of the astronomical season of spring. Almost simultaneously, Old Sol moves into the northern hemisphere of the sky, and the days become longer than nights. This event was perhaps the most important day in the civil and agricultural calendars of many Northern Hemisphere societies. In many cultures it marked the beginning of the new year. The Indian Civil Calendar still begins on March 22nd. From 1155 until 1751 the English Civil Calendar began on March 25th. The equinox also marks some of the most important dates in religious traditions. The Jewish celebration of Passover begins with the full Moon following the equinox, and the Christian celebration of Easter follows similar rules and fixes the dates of other feasts and observances throughout the rest of the year.

The term “equinox” means “Equal Night”; if the Sun were a point source of light, the length of day and night would indeed be the same on the 20th. However, the Sun subtends a half-degree disc, and sunrise occurs when the upper limb of the Sun crests the horizon, while sunset occurs when the limb disappears. Thus, the actual day when we see exactly 12 hours of daylight and night falls on the 17th.

The absence of the Moon in the evening sky means that it is time for the March observing campaign for the Globe at Night citizen science program. If you have any interest in the night sky, I encourage you to add your observations to the 20,000+ reports filed last year. The program aims to sensitize people to the spread of artificial light by measuring visible stars from around the world. From now until the evening of the 22nd look for the familiar constellation of Orion, which is prominent in the southwestern sky as evening twilight ends. Visit the Globe at Night’s Web App, and report your view of the Hunter with the provided star charts.

This is the time of year when we see one of the sky’s greater dramas play out. The constellations of Orion and Scorpius are related in Greek mythology, but you never see them in the sky at the same time. Orion was a formidable character, the son of the Gorgon Euryale and Poseidon, god of the sea. Other variations credit his origin to a humble shepherd, Hyrieus, who unwittingly entertained Zeus, Hermes, and Poseidon, offering the gods a feast of his ox, the only animal he possessed. The gods rewarded his generosity with a son born from the hide of the ox, who grew into the strapping Hunter. After ridding the island of Chios of ferocious beasts that overran it, Orion boasted that he would kill all of the wild animals on Earth. Gaia, the Earth goddess, was offended by this boast, so she sent a lowly scorpion to kill Orion. Both characters died in the ensuing battle, and Gaia placed them in the sky so that the scorpion would chase Orion for eternity. Look for Orion in the evening sky, then look to the southern horizon as morning twilight gathers to see Scorpius.

Venus continues to climb in the western sky as dusk falls. You should have no trouble spotting her bright glow almost immediately after sunset. My favorite time to view her is during deep twilight when the sky is a deep blue, fading into orange and red along the horizon. The sky seems to give Venus a slight blue tint, which becomes a stark white once twilight ends. Venus is undergoing a very favorable evening apparition which occurs every eight years. By late spring she will be high enough after evening twilight to cast a shadow at dark sky locations.

While Venus climbs, Jupiter descends toward the encroaching Sun. As the week begins you will find Old Jove about 12 degrees below Venus. By week’s end the gap between the pair increases to 20 degrees. Jupiter now sets at the end of evening twilight; by early April he is lost in the glare of the Sun.

Mars spends the week passing from the stars of Taurus to those of Gemini. His steady eastward pace can be measured by his increasing distance from the star El Nath, which marks the tip of Taurus’ “northern horn”. He still forms an attractive triangle with the ruddy stars Aldebaran and Betelgeuse, but the triangle’s shape now changes from night to night.

Chasing Orion

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