As darkness falls these next several evenings – August 20 to 22, 2018 – watch for the waxing gibbous moon to move by the planet Saturn and toward the red planet Mars. The moon travels about 1/2 degree (its own angular diameter) eastward per hour relative to the backdrop stars and planets of the zodiac. Keep in mind that the moon appears larger on the sky chart than it does in the real sky.
The moon goes full circle through the constellations of the zodiac every 27 and 1/3 days. This sort of month is called the sidereal month – one revolution of the moon relative to the backdrop stars. In contrast, the lunar month (or lunation or synodic month) is the time between successive returns to the same lunar phase, a period of about 29 and 1/2 days.
Although the moon goes by all the stars and planets of the zodiac every month, the moon’s phase will be somewhat smaller when the moon bypasses Saturn and Mars in September. And the month following will find the moon’s phase smaller yet when it meets up with Saturn and Mars in October. This happens because the sidereal month is some two days shorter than the lunar month.
It’s very handy for us at EarthSky to use the moon to point out key stars and planets of the zodiac on a monthly basis. But Mars is so close to Earth right now that you probably won’t need the moon to identify this brilliant ruddy world for months to come. Mars now beams as the fourth-brightest celestial object, surpassed only by the sun, moon and Venus.
Jupiter, which is usually the four-brightest heavenly body, now ranks fifth. But Jupiter will reclaim the fourth spot again, to trade places with Mars in the second week of September. Keep watching!
Venus and Jupiter are the brightest “stars” to light up the western half of sky, and pop out almost immediately after sunset. If you live at mid-northern latitudes, though, be sure to look for Venus fairly soon after sunset. Venus sets around nightfall from this part of the world; fortunately for The Southern Hemisphere, Venus stays out for a few hours after dark.
Given clear skies, you should be able to see four planets at nightfall: Venus, Mars, Jupiter and Saturn. Look westward for Venus and Jupiter, and then use the moon to find the planets Saturn and Mars from August 20 to 22, 2018.