The brilliant constellation Orion lies on the celestial equator (Earth’s equator projected onto the heavens), and so is visible—at one time of year or another—from every inhabited part of the globe. For viewers in northern latitudes, it dominates the winter sky. In many ancient cultures the constellation was associated with heroes or demigods. Among Europeans, Orion is the Great Hunter or Celestial Warrior. He is traditionally pictured in the stars with his club raised in his right hand and, hanging from his upraised left hand, the skin of a great lion that he has killed. Yet in contrast to such Greek mythical heroes as Hercules, who was credited with a detailed series of exploits, Orion is a vague and shadowy figure. Even the origin of his name is obscure.
The constellation, however, is perfectly clear, instantly recognizable from the row of three bright stars defining Orion’s belt. Two other stand-outs are the stars Rigel, in the imagined figure’s left leg, and Betelgeuse, marking the right shoulder. The two represent different periods in a star’s existence. Bluish Rigel, estimated to lie 770 light-years away from us, is a blazing hot supergiant, about seventy times the diameter of the Sun and shining roughly 57,000 times as brightly. It is reaching the prime of what will be a relatively short life of only a few million years (by comparison, the Sun is about 5 billion years old and should last another 5 billion). In contrast, reddish Betelgeuse, some 520 light-years away, is an irregularly pulsating supergiant, nearing the end of its expected life span of less than ten million years. It expands and contracts spasmodically, varying within the space of five or six years between 550 and 920 times the diameter of our Sun and shining about 85,000 times brighter.

The constellation also contains the Orion Nebula, a region of space where star formation is underway. To the naked eye it appears to be the centermost of the three stars that are envisioned as the Great Hunter’s sword, hanging from his belt. But a telescope will reveal the dim, diffuse spot as a fan-shaped greenish cloud of stars and interstellar gas. Edward Emerson Barnard (1857–1923), for many years an astronomer at Yerkes Observatory in Williams Bay, Wisconsin, once remarked that it reminded him of a great ghostly bat.

The nebula, which lies 1,344 light-years away, is about 30 light-years in diameter, or more than 20,000 times the diameter of the entire solar system. Its total mass (excluding stars) is 10,000 times that of the Sun, yet because it is so spread out, its density is less than a millionth that of the best laboratory vacuum. The Orion Nebula is largely visible to us through its fluorescence: high-temperature stars entangled within it emit strong ultraviolet radiation that triggers a sort of auroral glow.

March Nights Out

1 Two hours after sunset, look low in the eastern sky for the rising Moon. Well left of the Moon you’ll see a bright yellowish-white “star,” the planet Saturn. And about two-thirds up from the east-southeast horizon is yellowish-orange Mars, brilliant now but fading during the month as it gradually recedes from Earth.

3 Around this date some might report a bright “UFO” hovering just above the western horizon right after sundown. That object is brilliant Venus, which has been too close to the Sun to be seen since early December but is now setting one hour after the Sun.

7 The Moon wanes to last quarter at 10:42 a.m. eastern standard time.

14 Daylight saving time returns on this second Sunday in March. Clocks “spring forward” one hour at 2:00 a.m. local standard time.

15 The Moon is new at 5:01 p.m. eastern daylight time (EDT).

20 At 1:32 p.m. EDT the Sun appears to cross the celestial equator moving north. At the equinox, spring begins in the Northern Hemisphere and autumn in the Southern.

21 Saturn arrives at opposition to the Sun (at the opposite side of Earth from the Sun), rising around sunset and setting around sunrise.

23 The Moon waxes to first quarter at 7:00 a.m. EDT.

24 About forty-five minutes after sunset, look high in the southeast sky to find a nearly equilateral triangle formed by the Moon at the lower right, Mars at the lower left, and the star Pollux (the head of one of the Twins in Gemini) at the vertex.

26 About a half hour after sunset, using binoculars, scan near the west-northwest horizon to locate Mercury in the bright evening twilight. In the coming days, the planet will linger longer after sunset, making it easier to see even though it is fading in brightness.

29 The Moon is full at 10:25 p.m. EDT.