Tonight – January 29, 2017 – look westward after sunset to view the two brightest luminaries of nighttime, the moon and Venus. Be sure to catch the moon as soon as darkness falls. The slender waxing crescent moon will sit rather low in the sky, and will follow the sun beneath the horizon by early evening.
Day by day, you’ll see a wider waxing crescent moon higher up in the sky at sunset and staying out longer after dark. That’s because the moon is presently moving away from the setting sun and toward the planet Venus on the sky’s dome. Tomorrow – after sunset on January 30 – look for the moon to be closer to Venus in the evening twilight sky.
The moon always goes westward (toward the sunset) each day, due to the Earth’s spinning motion from west-to-east on its rotational axis. Yet, the moon is actually moving eastward with respect to the background stars and planets of the zodiac, due to its motion in orbit around Earth. As you watch the moon’s change of position over several days, as depicted on the sky chart above, you can begin to get a sense of the moon’s orbital motion around Earth.
That motion causes the moon to move eastward by about 1/2 degree (the moon’s own apparent diameter) in one hour, or 12o eastward per day, with respect to the sun. In contrast, the moon travels about 13o eastward per day as measured by the backdrop stars.
Or, to put it another way, the moon goes full circle relative to the backdrop stars of the zodiac in about 27.3 days and full circle relative to the sun in about 29.5 days.
Bottom line: Beginning on January 29, 2017, watch as the waxing crescent moon sweeps past Venus and then Mars on the sky’s dome. You’ll find them all in the west at nightfall.
Bruce McClure has served as lead writer for EarthSky's popular Tonight pages since 2004. He's a sundial aficionado, whose love for the heavens has taken him to Lake Titicaca in Bolivia and sailing in the North Atlantic, where he earned his celestial navigation certificate through the School of Ocean Sailing and Navigation. He also writes and hosts public astronomy programs and planetarium programs in and around his home in upstate New York.