Wednesday, September 26, 2007

Today is the day of this year's Harvest Moon. The exact time of the event is when the Moon reaches the spot in the sky where it is precisely opposite the Sun. This will occur at 19:45 Universal/Greenwich time today (7:45 pm). This will be 14:45 (2:45 pm) here in Central Daylight Time. To find the timing for your own time zone see the Time and Date website.
All of the full moons of the year have popular names that differ from place to place. See HERE, HERE and HERE for three different lists. Other names for the Harvest Moon include the Wine Moon, the Singing Moon, the barley Moon, the Fruit Moon and the Elk Call Moon.
The Harvest Moon is the most common name. The Harvest Moon is defined as the full moon that falls closest to the Fall Equinox which occurred three days ago on September 23rd. This moon can actually occur earlier than the equinox, and in about one year out of four it occurs in October. This happened last year but will not occur again until 2011.
The popular belief that the Harvest Moon stays in the sky longer than other full moons is false. But farmers traditionally did have more time to work by moonlight at this time of year. This is because of the variation in time of moonrise from day to day as the seasons change. Usually the Moon rises about 50 minutes later each day. When the Moon is, however, travelling in the sky near the path of the ecliptic, as it does in September, October and November, the day-to-day difference is foreshortened. The effect depends upon latitude. The further north you are the shorter the daily difference between moonrises. In Miami, for instance, the Moon rises an average of 37 minutes later from day to day at this time of year. At the latitude of Edmonton, Alberta, however, the average difference is only 12 minutes. This meant that on successive nights during the harvest season that farmers would indeed have more time to work the fields by moonlight on each day of the harvest. The same is true for the next moon of the year, usually known as the Hunter's Moon. In the southern hemisphere the situation is reversed, and the Moon rises more than 50 minutes later from day to day at this time of year.
The Harvest Moon is also neither brighter nor of a different colour than other full moons of the year. People who go out early to observe moonrise around the time of the Harvest Moon may be fooled by what is known as the 'Moon Illusion' into thinking that the Harvest Moon is bigger. Any moon, or any other sky object for that matter, always looks bigger when it is near the horizon. This is despite the fact that it occupies exactly the same angular diameter in the sky. The difference in size is a trick of perception rather than any real physical effect. Similarly the rising Harvest Moon is no redder than any other rising Moon. Refraction of light due to Earth's atmosphere gives all objects near the horizon a reddish hue. Think of the rising or setting Sun.
As mentioned previously on Molly's Blog (See 'Welcome to Autumn' last weekend) today is also the day of the equilux. This is the time when daylight and nighttime are most exactly equal. There will be another such day next spring. Til then...

No comments: