The Moon and Planets

The Moon reaches its Last Quarter phase in the wee hours of Wednesday morning. Last quarter moons rise about midnight and persist into the daytime morning sky, leaving the evenings darker for stargazing. The 90° angle made between the Sun, Earth, and Moon at last quarter cause the moon to be half illuminated, lit on the eastern (left-hand) side. The terms gibbous and crescent are used to describe objects that are more than half illuminated, or less than half. By the way, only bodies that venture between us and the Sun can ever appear as crescents when viewed from the surface of the Earth. Those are the Moon, Venus, and Mercury (and some asteroids).

The day before last quarter, from midnight to dawn on Tuesday, September 12, the waning gibbous moon will pass through the stars marking the triangular face of Taurus (the Bull). As dawn approaches, the moon moves towards Taurus’ brightest star Aldebaran. You can try using a backyard telescope to see the Moon pass in front of (or occult) the star. In the Great Lakes region, the Moon covers Aldebaran around 8:48 am EDT and moves off it about 10 am (times varies by location). Observers in western North America and Hawaii will see the event in a dark sky.

For the rest of the week, the Moon wanes and rises later, and passing though the legs of Gemini (the Twins) and then appearing as a thin crescent above Venus in the eastern pre-dawn sky on Sunday morning.

This week, Mercury is giving us its best appearance of the year for mid-northern skywatchers all over the world, and it’s easily seen with plain old eyeballs. Look for it very low in the eastern sky from the time it rises, about 5:30 am local time, until about 6:30 am. On Tuesday morning, the planet will reach its widest angle west of the sun, and peak visibility. If you want to try using a telescope, Mercury will exhibit a waxing half-illuminated phase. But be sure to point the telescope away from the eastern horizon well before the Sun comes up.

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