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Sky Roundup for September 2010
Astronomical highlights for the month of September
Shine On, Harvest Moon
Just over a hundred years ago, a catchy vaudeville tune, “Shine on, Harvest Moon”, debuted and became a pop standard for many decades thereafter, right into the 21st century, featured as it was by recording artists and by its inclusion in popular movies.
The lyrics start out with a forlorn lament of a loveless Winter, Spring, and Summer but end in anticipation of a cheery Autumn, if only with a little romantic help from the Harvest Moon. The cover for the original sheet music (seen below) sets the scene for the song’s upbeat ending, showing two pining lovers, together at last, silhouetted by the rising light of that familiar orange orb.
While the cover was prepared in the design style of the day, the publisher of the sheet music captured several classic images reminiscent of the approaching season. Perhaps no actual attempt was made to be astronomically correct; after all, they were trying to sell sheet music. Rather, it may be more likely that the publisher was aware enough of the change of seasons to include those images that best describe it.
As Summer draws to its astronomical close this month, we see the iconic signs of its passing. If you live in or near rural parts of the Midwest, you’ll see the start of the corn harvest by the end of this month. While farmers no longer arrange harvested corn in shocks as shown in the sheet music cover (---they arranged it in shocks to dry or cure the corn---), many homes still decorate with them to represent the season. Even the famed candy corn, popular at Hallowe’en, has the iconic shape of the curing corn shock.
From the less romantic side of things, the science of astronomy makes clear the beginning of the season, whether we like it or not, and its start does not necessarily match the weather from day to day. Indeed, our modern calendar appears to mark the start of seasons after we are well into the weather that typifies them.
For example, the astronomical start of Summer (as the Summer Solstice) shows the Sun at its highest in the Northern Hemisphere, though that season's weather already is fairly warm by that date. On older calendars, that date actually marked the middle of the season, as in William Shakespeare’s “A Midsummer Night’s Dream”.
Similarly, Autumn begins this month, specifically on September 22, at 10:09 p.m. CDT, which marks the astronomical start of the season, the Autumnal Equinox. But by the feel of the weather, it may make more sense that the date ought to be the mid-point of the season.
However, since at this point in Earth’s orbit, the rotational axis of Earth makes a right angle to an imaginary line joining the center of Earth and the center of the Sun, this is actually Autumn’s beginning.
As seen across the Midwest at harvest time and typical of this season, too, is often the rising Moon, large and orange near the horizon, just like the cover of the sheet music. From our sky perspective, the music publisher got it right.
The Moon can appear large due to the angle at which we see it. If near the horizon, the Moon appears close to objects along the horizon, such as trees, houses, fence posts, and even corn shocks. Seen near these landmarks in the foreground, the Moon appears huge in the background; away from these familiar landmarks, we have no point of reference, so the Moon appears small. In reality, the Moon does not change size; it’s just an illusion of large size that everyone sees.
The Moon can also appear orange due to the air around us. Near the horizon, we spy the Moon at a shallow angle, and, through more of the atmosphere, which filters out most of its reflected light except for longer wavelengths at the red end of the spectrum, making that orb look orange from our perspective.
You can easily find any number of websites that may feature copyrighted photo galleries of a large, orange Moon. In Autumn, we’re more accustomed to seeing the Moon large and orange because it seems to dawdle along the horizon. In actuality, the low angle of its orbit to the horizon makes the Moon appear to glide low over treetops as it rises.
So, of course, the best-known sign of the approaching season of Autumn indeed is the title “Harvest Moon”. That’s the name given to the Moon that occurs nearest to the Autumnal Equinox. At the time that the Harvest Moon song was first popular, it was a quaint notion that farmers relied on that light of the rising Moon to finish their harvest for the day. Nowadays, farmers finish in the field more efficiently with modern machinery, but it’s not an old-fashioned notion that Earth’s nearest astronomical neighbor still influences our daily life.
The Moon can certainly influence what we see this month, since it shines next to most of the planets. On September 10th, a slim, waxing crescent hugs the horizon just below Venus, Mars, Saturn, all in the constellation Virgo (the Maiden). Of course, Venus is far brightest of the trio. At the start of the month, Mars and Saturn are barely visible in early evening twilight as one will need a telescope of modest magnification to spot either of them at all. (See Chart 1. Saturn is very close to the western horizon.)
By mid-month, the famous ringed planet has bowed out and is lost in evening twilight. By the end of the month, Mars starts to go the way of Saturn.
Less than two weeks later, on September 23rd, the Full Moon can be spotted above Jupiter and Uranus in Pisces (the Fishes) all through the night. Of the two, faraway Jupiter by far outshines more distant Uranus, which will require a good telescope’s high magnification just to spot it as a tiny greenish dot. (See Chart 2. Note the reddish tick marks to help pinpoint the location of Uranus.)
If you use low power, such as in a large binocular, look at Jupiter and then look for Uranus nearby as both of these distant planets will be very nearly in the same field of view!
In September, Jupiter appears brightest and in its most favorable apparition when it reaches opposition (that is, when it appears opposite the Sun in the sky) this month. Finding Jupiter is easy in the eastern sky as it rises earlier every evening and is the brightest object in the sky all night long (besides the Moon) after Venus sets.
For this month, Mercury remains in the glare of the Sun, emerges into morning twilight my mid-month, but is not well-placed for observing at all until the end of September.
For the Moon itself, it’s always an appealing subject, when one can watch the change of phases throughout the month. No matter where one lives, in the city, in the suburbs, or in the country, the Moon, when visible on clear nights, commands the evening sky. And in some urban areas where often only the Moon can be seen, it’s worth taking time to observe it.
Look at the pictures (below) of typical lunar profiles. Observe a usual 6 day-old Moon, the waxing crescent, and you’ll catch the terminator, that shadowed divide between light and dark on the Moon, highlighting craters and mountains in bold relief.
Zoom in with medium magnification, say, greater than 60-power, and scan the deep valleys and sheer crater walls.
Look later for the gibbous phases before and after Full Moon, namely, waxing at 9 days and waning at 18 days, and note the change of perspective simply by shifting the Sun’s light across ancient landscapes.
Some observers might think that seeing the same set of landscapes month after month would get tiresome or boring. But the play of light and shadow across the lunar surface never ceases to fascinate.
Shine on, Harvest Moon!
Sky Roundup Almanac for Sept 2010
September 1:
Last Quarter Moon
September 8:
New Moon
September 15:
First Quarter Moon
September 21:
Jupiter reaches opposition and is visible in the eastern sky after sunset
September 22:
Autumnal Equinox, first day of Fall, at 10:09 p.m. CDT
September 23:
Full Moon
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Almanac source: U. S. Naval Observatory
All almanac events corrected to Central Daylight Time.
Charts prepared with the aid of the Stellarium program.
