Monday evening bring us a first quarter moon, offering U.S. skywatchers a chance to close out the Memorial Day holiday with a lunar treat and the fading planet Mars.
The first quarter moon is also sometimes called the “half” moon, although as darkness falls across North America close inspection with binoculars or a small telescope may reveal that there is actually a little more than half of the moon illuminated by the sun. That’s because the actual moment of first quarter occurred earlier in the afternoon at 4:16 p.m. EDT or 1:16 p.m. PDT (2016 GMT).
When it gets sufficiently dark you’ll also take notice of a rather bright yellow-orange star appearing well above and a bit to the moon’s left. That’s not a star, however, but a once brilliant planet that continues to ebb in brightness: Mars. Look for this colorful world shining high in the southwest during dusk and lower in the west-southwest as night grows late.
Mars will remain prominent all evening, despite the fact that it will continue to slowly fade in the days and weeks to come as it gradually pulls away from the Earth. When the moon passes it by on Monday, it will still be shining at a respectable magnitude +0.5, which is just a trifle dimmer than the ruddy star Betelgeuse in Orion.
Astronomers measure the brightness of objects in the sky on a magnitude scale. The lower an object’s magnitude number, the brighter it appears in the night sky. Small digits and negative number magnitudes denote the brightest night sky objects.
By the end of June, Mars will have faded fourth-tenths of a magnitude, to +0.9 (remember the higher the number, the fainter the object), making it just a bit brighter than Spica, the brightest star in the Virgo constellation. By that time its position will be about midway between Spica and the planet Saturn to its east and the bluish 1st-magnitude star Regulus in Leo to its west.