Sky ArchiveTonight

Moon below Venus on January 29

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.

The 27.3-day lunar cycle is called the sidereal month, while the 29.5-day cycle of the phases is called the lunar or synodic month.

Click here for an illustration of the synodic month

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.

January 29, 2017
Sky Archive

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Bruce McClure

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