In the Sky This Month

The Moon is at first quarter today, exactly one-quarter of the way through its month-long cycle of phases. It lines up at a right angle to Earth and the Sun, so sunlight illuminates half of the lunar hemisphere that faces our way.

The Orion Nebula, a stellar nursery, hangs below the hunter’s belt at nightfall. A binary star known as Iota Orionis is near the nebula. An encounter with another binary may have kicked one star from each system out into space, with the remaining stars forming a new binary.

Aldebaran, the bright eye of the bull, stands just below the Moon at nightfall, and leads the Moon down the sky later on. Aldebaran is just 65 light-years away. It is a bloated star near the end of its life.

The constellation Coma Berenices, which represents the hair of a legendary queen, is well up in the east by mid-evening. It is about half way between the bright stars Arcturus and Regulus, which highlight the eastern sky.

Spring arrives in the northern hemisphere today, but the most prominent stars of winter remain in good view. They form a big loop known as the Winter Circle, which is in the southwestern quadrant of the sky this evening.

The Sun will cross the celestial equator tomorrow. The crossing marks the vernal equinox, which is the beginning of spring in the northern hemisphere. It also marks the starting point for measuring the length of the year.

The vanishingly thin crescent Moon has a couple of companions after sunset this evening, the planets Venus and Mercury. Venus is the “evening star,” to the right of the Moon. Much-fainter Mercury is about the same distance to the upper right of Venus.

The Owl Nebula stares out from the Big Dipper. It is a set of concentric bubbles of gas blown into space by a dying star. It’s round, and seen through a telescope or in photographs, it has two dark patches that look like an owl’s eyes.

The Moon will be “new” early tomorrow as it crosses the line between Earth and the Sun. It is lost from sight in the Sun’s glare, but should return to view on Sunday, as a thin crescent quite low in the west at sunset.

One of spring’s most prominent stars is Arcturus, in the constellation Bootes, the herdsman. This yellow-orange star rises in the middle of the evening and soars high across the sky during the night.