Thursday, September 11, 2008

Shine On, Harvest Moon

The Harvest Moon is the Full Moon that occurs nearest the autumnal equinox, also known as the fall equinox. The Harvest Moon usually occurs in September and occasionally in October. This year, the Harvest Moon falls on Monday, September 15, one week prior to the fall equinox on Monday, September 22.

The fall equinox marks our official first day of autumn. Astronomically speaking, the equinox (EE-kwih-nocks) is the moment in time, calculated to the minute, when the center of the Sun is above the Earth’s equator. This occurs only twice a year, in the spring and in the fall.

To understand why equinoxes occur only twice a year, you need to remember that Earth’s axis is tilted 23.5 degrees with respect to its orbit around the Sun. If it weren't, then Earth’s equator would line up with the plane of Earth’s orbit around the Sun, and the center of the Sun would always be above the equator. Of course, we would have no seasons. The temperatures at a given location on the globe would not vary much over the course of a year. We wouldn’t see the Sun arcing high in the sky in the summer or arcing low--nearer the southern horizon--in the winter. It would pretty much follow the same course in the sky, sunrise to sunset, day after day, all year long.

Earth’s tilt with respect to its orbit around the Sun
Image courtesy of Tau’olunga

But that's simply not the case, is it? Our 23.5-degrees-off-kilter planet ensures that here in the Northern Hemisphere we tilt toward the Sun in the summer and experience the heating effect of direct sunlight, and then tilt away from the Sun in the winter and get the chilling effect of indirect sunlight. In spring and fall, midway between the two temperature range (and tilt) extremes, we enjoy moderate temperatures.

Another way to think of the equinox is that it’s when the ecliptic (the imaginary line that represents the path the Sun appears to take across the sky, as seen from Earth) intersects with the celestial equator (the imaginary line that represents the plane of the Earth’s equator, extended out into space). The diagram below shows the twice-yearly intersection of these two lines at the equinoxes.

A Tale of Two Planes: Ecliptic and Celestial Equator
Diagram by
Dr. Guy Worthey

The Earth’s sunward tilt in the summer puts the plane of the equator below the Sun’s center (which corresponds to the ecliptic), and the Earth’s sun-shunning tilt in the winter puts the plane of the equator above the Sun's center. Only during the in-between seasons of Earth’s orbit--spring and fall--can the Sun’s center intersect with the celestial equator and therefore be directly above the Earth’s equator.

The Harvest Moon was so named because it occurs during the traditional peak of crop harvesting in the Northern Hemisphere. In times past, farmers rushing to bring in their crops could continue working by moonlight after the sun had set.

The Harvest Moon was particularly revered by hard-working farmers of old for a reason that has astronomy at its root. Normally, the Moon rises around 50 minutes later each successive night. However, for several days around Harvest Moon, the Moon rises only 25 to 30 minutes later than the evening before. This eliminates or shortens the period of darkness between sunset and moonrise that might otherwise hamper farmers still out standing in their fields. These quicker-than-usual moonrises are so noticeable they’ve given rise to a mistaken but persistent belief that the Moon rises at the same time on successive evenings around the Harvest Moon.

The astronomical reason for the shortened time between moonrises around Harvest Moon is that the angle between the ecliptic and the horizon is very narrow in September. Contrast this with the beginning of spring--in March--when the angle is very steep. The Moon appears to follow approximately the same path across the sky as the Sun, that is, the ecliptic. As the Moon moves along the ecliptic from night to night in September, its shallow angle relative to the horizon ensures that the turning Earth will reach the Moon’s new position on the ecliptic sooner than it would if the ecliptic were steeply oriented to our horizon.

A picture is worth a thousand words, so take a look at this diagram by Patrick Moore, comparing the ecliptic’s angle relative to the horizon in both spring and fall.

The old Anglo-Saxon word for autumn, “hoerfest,” gave us our mouth-watering word “harvest.” Thoughts of the harvest naturally turn to thoughts of food and drink. The Chinese Mid-Autumn Festival, a harvest celebration which occurs near the autumnal equinox, falls this year on Sunday, September 14. The festival includes the ritual eating of mooncakes, round pastries traditionally filled with lotus seed paste and sometimes a salted egg yolk to symbolize the Full Moon. If you don’t have an Asian market offering mooncakes in your area, try this modern recipe for mooncakes.

If mooncakes aren’t your cup of tea, you can always fall back on an American classic: the MoonPie. Developed in 1917 as a snack for coal miners, it was purposely fashioned to look like the Full Moon (and therefore, substantial). The marshmallow and graham cracker treat is available with a chocolate, vanilla, or banana-flavored coating.

For a walk on the wild side, try this Harvest Moon cocktail. As for me, I’ll be toasting in the Harvest Moon with a tall glass of ice-cold apple cider.