A Full Corn Moon to Marvel at, and a Pre-dawn Planet Party with Mercury!
The Full Moon
The full moon that occurs nearest to the Autumnal Equinox is called the Harvest Moon. That usually happens in September, but this year that falls in October. North American First Nations call the September full moon the Full Corn or Full Barley Moon. In Chinese, full moons are called 望月(Wàngyuè). The Moon is only 100% full at a precise moment, specifially 3:03 am on Wednesday morning, September 6, so it will seem to be full on both Tuesday and Wednesday evenings.
A closer look on Tuesday evening will reveal that a sliver along the Moon’s left edge is still dark. The sliver shifts to the right edge on Wednesday evening. (Try using binoculars to see it!) This happens because full moons occur only when the Moon is at opposition — the precise moment when the Earth sits on the imaginary line connecting the centres of the Sun and the Moon. While the Sun-Earth-Moon alignment happens every month in a “left-right” or “east-west” sense, the three bodies rarely align in the “up-down” sense at the same time. This is because the Moon’s orbit is tilted by 5° compared to the Earth’s orbit. When the perfect alignment does happen, a Total Solar Eclipse or Total Lunar Eclipse occurs. After Wednesday, the Moon will rise later and later, and start to wane.
The September full moon always shines in or near the stars of Aquarius (the Water-Bearer) and Pisces (the Fishes). The Moon’s continuous eastward orbit shifts it along the ecliptic by about 12° (a generous fist’s diameter) every night. For most of the year, this makes the Moon rise about 50 minutes later every night. The autumn fall moons tend to catch your eye more because the evening ecliptic is shallow this time of year and the moon’s rise is delayed by only 30 minutes — making it likely you’ll see it on your regular evening stroll or commute home for two or three nights in a row.
During full moon, the Sun’s light is shining directly onto its surface as seen from our vantage point on Earth. This means that if we turn our face to a full moon, the back of our head always points towards the Sun. For this reason, full moons always rise in the East as the Sun is setting in the West. Astronomers don’t like full moons very much because they flood the sky with scattered moonlight that washes out the faint stars and objects. And full moons aren’t as interesting in a telescope because none of the lunar topography is casting shadows. You might as well just look at a poster on the wall!
Full moon gazing with unaided eyes is fun. Some people see a woman’s face — showing her left profile and wearing a bright pendant at her neck (the crater Tycho). Some see a jumping rabbit formed out of the dark areas across. The Moon’s dark areas are composed of a dark, heavy, and coarse grained rock called basalt, common around volcanoes here on Earth. We call the areas Maria (Latin for “sea”) because they are vast, low lying regions that have been flooded and filled with basalt from long gone lunar volcanoes, forming flat “seas” of rock. Many of the Apollo missions landed in them to ensure that a rugged or uneven surface feature didn’t damage or tip over the Lunar Excursion Modules (LEM).
Apollo 11 landed in Mare Tranquilitatis (the Sea of Tranquility), the “hair” just above the lady’s ear. The Apollo 12 and Apollo 14 sites are both just to our left of her chin. Later missions became bolder. Apollo 16 landed in the bright lunar highlands, just about where the lady’s earring would be! Finally, the isolated bright spot in front of her face is the famous crater Copernicus. Its impactor seems to have punched a hole through the dark basalt to expose the brighter rocks underneath (also true for Tycho and its long rays of debris). Go out and take a look!
The Moon and Planets
Mercury rejoins the morning pre-dawn sky this week for the best appearance of the year for North American skywatchers. On Monday morning, you can try to spot Mercury very low in the eastern sky, sitting about three finger widths to the right of Mars and the bright star Regulus. The best time to look is from 5:50 to 6:10 am local time — but put the binoculars away before the Sun rises! Every morning thereafter, Mercury gets easier to see as it brightens and climbs higher while shifting closer to Regulus (but leaving Mars behind). By the weekend, the optimal looking times are between 5:35 and 6:10 am local time.
While you’re outside, why not enjoy extremely bright Venus. It’s much higher than Mercury because it rises in the eastern sky just after 4 am local time. This week, the planet continues to descend slowly sunward, leaving Cancer (the Crab) and approaching Leo (the Lion).
After missing for months, Mars re-appears in the pre-dawn sky this week, rising about 5:35 am local time and slowly drawing away from the Sun. But it’ll be some time yet before it becomes a nice target. I’ll post sky charts for the visible morning planets here.
Jupiter is the bright white object low in the southwestern evening sky for 90 minutes after sunset. It sets about 9 pm local time this week. Saturn is the medium-bright, yellowish object partway up the southern sky as the evening darkens. It sets in the west about midnight local time. Saturn’s moons can also be seen in a small telescope. They can sit above, below, or to either side of the planet and venture up to many ring diameters from it.
Blue-green Uranus, situated in Pisces (the Fishes) rises after 9 pm local time and is observable for the rest of the night in binoculars under a dark sky (but not this week!). On Tuesday, September 5, tiny blue Neptune will be directly opposite the Sun in the sky, putting it at its closest and brightest for this year. Also observable all night, it is located about two finger widths to the lower left of the medium-bright star Hydor in Aquarius (the Water-Bearer). It is too faint to be seen with the unaided eye, especially with the bright moonlight this week.
Stargazing News for this week (from September 3rd, 2017) by Chris Vaughan.