Saturday finds the moon in a waxing gibbous phase.
Of all the phases that the moon goes through over 29.5 days, a waxing gibbous is probably the most frequently observed. The word gibbous is derived from the Latin gibbus, which literally means “hump.” An uncommon word to be sure, but in describing the shape of the moon now, it is quite correct.
On this particular occasion, almost three-quarters of the moon’s disk (74% to be exact) will be illuminated by the sun. And the moon makes itself evident well before sunset. It will rise around noontime and by 4 p.m. it will be visible well up in the eastern part of the late afternoon sky and will remain in view until nearly the break of dawn the following morning.
Moon joins a planet and two stars
When it gets sufficiently dark tonight, you’ll also take notice of a rather bright yellow-orange star appearing just below the moon. That’s not a star, however, but a once brilliant planet that continues to diminish in brightness: Mars.
Look for this colorful world soaring high in the south-southeast during dusk and lower in the west-southwest as the night grows late. At this moment, it forms a conspicuous triangle with two nearby bright stars, Castor and Pollux, in the constellation Gemini. To echo H.A. Rey in his classic “The Stars — A New Way to See Them”:
“If the moon and a planet are near Castor and Pollux you have an impressive show.”
From our earthly perspective, the moon and Mars will appear closest to each other at around 8 p.m. Eastern Time, with Mars slipping about 1.2 degrees beneath the moon’s lower limb. Since the moon measures roughly one-half degree across, you would think that Mars would appear more than two moon widths from the moon’s lower limb. But because the moon appears to our eyes to be twice as large compared to its actual angular size — an optical illusion — moon and planet will actually appear about half their predicted distance apart.
As Mars withdraws . . . its brightness ebbs
Mars will remain prominent all evening, despite the fact that it will continue to fade slowly in the days and weeks to come as it gradually pulls away from the Earth. When the moon passes it by on Saturday, it will still be shining at a very respectable magnitude -0.1, which makes it a match both in brightness and orange color to Arcturus in Boötes, the fourth brightest star in the sky. However, back in January, Mars was nearly a match for Sirius, the brightest of all stars; currently, Mars shines only one-third as bright.
But by the end of March, as a direct consequence of its recession from Earth — an average of 583,000 mi (939,000 km) per day — Mars will have faded five-tenths of a magnitude, down to +0.4 (the higher the value, the fainter the object), making it about equal to yellow-white Procyon, the brightest star of Canis Minor. By then its position will have changed so that it is no longer forming a triangle with Pollux and Castor, but rather a crooked line.
On April 12, Mars will leave the boundaries of Gemini, where it has resided since the second week of January and move into the much dimmer zodiacal constellation of Cancer the Crab.
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Because Mars soon comes to eastern quadrature — 90 degrees east of the sun — on April 20, it too will appear distinctly gibbous in even a very small telescope. By then, most amateur telescopes won’t be showing any substantial detail on Mars; in fact, by the end of April its apparent diameter will have shrunk to 6.6 arc seconds, which is less than half the size Mars appeared to us in mid-January when it was closest to Earth at 59.5 million mi (95.7 million km) away. In contrast, by April 30, Mars will have receded to a distance of 132 million mi (212.4 million km) from us.
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