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This Week’s Sky: January 13 to 19

Waning gibbous Moon, the last Quarter Moon, Algol reaches minimum brightness and the waning crescent Moon near Mars.

This week’s night sky highlights include the waning gibbous Moon positioning near Regulus, the last Quarter Moon, and Algol reaching minimum brightness as its dim companion star crosses in front of the much brighter main star. To finish the week, Mars and the Moon will move so close together they can both fit into the field of view of binoculars.

Monday, January 13 pre-dawn: Bright Moon near bright Regulus 

In the western pre-dawn sky on Monday, January 13, the waning gibbous Moon will appear within three finger widths to the upper right (or 3 degrees to the celestial north) of the bright, white star Regulus, which marks the heart of Leo, the Lion. Also designated Alpha Leonis, Regulus sits within one degree of the ecliptic (green line) and is occasionally occulted by the Moon and planets.

Friday, January 17 at 12:58 GMT: Last Quarter Moon

At its last quarter phase, the Moon rises around midnight and remains visible in the southern sky all morning. At this phase, the Moon is illuminated on its western side, towards the pre-dawn Sunday. Last quarter Moons are positioned ahead of the Earth in our trip around the Sun. About 3½ hours later, Earth will occupy that same location in space. After this phase, the waning Moon will traverse the last quarter of its orbit around the earth, on the way to new Moon.

Sunday, January 19 at 8:21 p.m. EST: Algol at minimum brightness

The “Demon Star” Algol in Perseus is among the most accessible variable stars for beginner skywatchers. Its naked-eye brightness dims noticeably for about 10 hours once every two days, 20 hours, and 49 minutes because a dim companion star orbiting nearly edge-on to Earth crosses in front of the much brighter main star, dimming its light. On Sunday, January 19 at 8:21 p.m. EST, Algol will reach its minimum brightness of magnitude 3.4. At that time, for observers in the Eastern time zone, it will sit close to the zenith in the southwestern sky. Five hours later, at 1:21 a.m. EST, Algol will be over the northwestern horizon and will have brightened to its usual magnitude of 2.1.

Monday, January 20 pre-dawn: Old Moon near Mars

In the southeastern sky during the hours preceding dawn on Monday, January 20, the waning crescent Moon will be positioned less than four finger widths to the upper right (or 4 degrees to the celestial northwest) of Mars. Both objects will fit into the field of view of binoculars.