The Moon and Planets

The moon continues to grace the evening sky this week, reaching its First Quarter phase around noon today (Sunday). The moon’s four main phases — New, First Quarter, Full, and Last Quarter — each occur at the precise moment when the required relative positions of the Sun, Earth, and moon are correct. Because the Earth’s rotation is not connected to any of that, those phases can occur at any time of day or night. For example, a full moon that happens at noon will look full that evening and also on the evening before. Although in actuality, the moon will be slightly past full and slightly before full respectively.

Keep the binoculars (or telescope) handy all week, because the waxing moon is gorgeous at this time of its cycle. Peaks and craters catching the sun’s rays of light cast deep shadows along the terminator, the boundary separating the lit and dark sides of the moon. Zooming in on the terminator, and scanning it from the moon’s north pole to the south, is a lot of fun. And new areas are revealed every night!

(Above: During the course of the night on Saturday, December 2 and into Sunday morning, the moon’s orbit will carry it eastwards into the triangular face of Taurus the Bull. For observers in the west, the moon will occult the bright star Aldebaran before moonset. The Earth’s rotation also causes celestial objects to rotate clockwise, as can be seen by Taurus’ orientation change.) Image via Star Walk app.

Overnight this Saturday, you can see the moon’s orbital motion in action. When the nearly full moon rises in the east after sunset, it will be sitting about a palm’s width to the upper right (west) of the stars that form the triangular face of Taurus. The bright reddish star Aldebaran will occupy the lower left corner of the triangle, and the sparkly little Pleiades Cluster (also known as M45) will be about one clenched fist’s diameter above the moon. Each hour, the moon’s orbit will carry it one full moon diameter (or 0.5°) eastwards. By midnight, the moon will be approaching the “chin” star. And just before dawn, before it sets in the west, the moon will be in the centre of the bull’s face. A few hours later, observers in Central and northern Asia, northern Greenland, and northwestern North America will get to see the moon pass in front of (occult) Aldebaran.

Next Sunday brings us the December full moon, traditionally known as the Oak Moon, Cold Moon, Long Nights Moon, and the Moon before Yule. It rises at sunset and sets at sunrise and, because the full phase occurs at 10:47 am EST, it will look full on both Saturday and Sunday night. The position of the ecliptic on winter nights causes December moons to culminate very high in the night sky, casting shadows that match with summer noon sunshine! This full moon occurs less than one day before perigee, the point in the moon’s orbit when it is closest to Earth, making Sunday’s full moon the largest (about 7% larger than average) and brightest for 2017, sometimes referred to as a supermoon. We’ll also experience extra high tides globally because the closer moon’s gravity will pull more strongly on the Earth.

(Above: Mercury has reached the outer turn of its orbit and now starts swinging sunward again. For the early part of this week, look for Saturn sitting a few degrees above the innermost planet. Sky shown here at 5:15 pm local time on November 26.) Image via Star Walk app.

A few days ago, Mercury reached its widest separation from the sun, and peak visibility, for this appearance. Now it is swinging sunward again, but you still might catch a glimpse of it low in the western sky around 5:15 pm local time. For the next few days, Mercury will also sit a few degrees below Saturn.The ringed planet is well past its prime now, embedded in the evening twilight and setting about 6 pm local time (half an hour after sunset). This week gives us the last opportunity to see Saturn until it returns in the pre-dawn sky after solar conjunction on December 21. Use binoculars to hunt for these two planets, but be careful to avoid the sun!

Blue-green Uranus is midway between the two chains of stars that form the dim constellation of Pisces (the Fishes). By the time the sky is dark enough to hunt for it, it’s about 3.5 fist diameters (or 35°) above the eastern horizon. It remains observable for the rest of the night as it crosses the sky. To help guide you, a medium-bright star called Omicron Piscium sits about 2.5 finger widths to the lower left of Uranus and another star of comparable brightness called Mu Piscium is three fingers to the planet’s lower right. (Remember that the sky rotates through the night, so this triangle of two stars plus Uranus will be tipped towards the west if you look later in the evening.)

Tiny blue Neptune is located in the southern evening sky about half a finger width to the lower right of the medium-bright star Hydor in Aquarius (the Water-Bearer). It is observable from full darkness until about midnight local time, but it’s too faint to see without a telescope.

(Above: The naked eye planets Mars, Jupiter, and Venus, shown here at 6:45 am local time, are spaced out along the morning Ecliptic, which defines the plane of our Solar System. Mars and Jupiter are heading towards one another and will be visible for months, while descending Venus is days away from vanishing into conjunction with the sun.) Image via Star Walk app.

The eastern pre-dawn sky is where most of the showy planetary action is taking place this month — with a string of visible planets showing how the plane of our Solar System sits! Dim, red-tinted, Mars rises first, around 3:40 am local time. This week, it is sitting high in the pre-dawn sky, about a palm’s width above the bright star Spica in Virgo (the Maiden). Over the course of this week, Mars’ eastward orbital motion will carry it lower while Spica and the rest of the stars move higher, so their relative positions will change.

At the same time, very bright Jupiter, which rises about 5:15 am local time, is climbing slowly higher, so the separation between Mars and Jupiter will diminish steadily, until they “kiss” in early January! Finally, very bright Venus, is mirroring Saturn. It’s embedded in the eastern dawn twilight for about half an hour before sunrise.

Astronomy Skylights for this week (from November 26th, 2017) by Chris Vaughan.

Keep looking up to enjoy the sky!