Natural light was a highly valued commodity in the northern parts of the world when crops had to be gathered from the fields before the first frosts. The equinox moon was exactly what was needed. It provided "extra" light, allowing workers to harvest into the evening hours long after the sun had set.

In most years, the Harvest Moon is the full moon nearest to the autumnal equinox. Sometimes, however, the honor goes to October, as it did in 1990. At that time, the moon was full on the fourth day in both September and October, hence, the full moon nearest the equinox September 22 was the October 4th moon.

The Harvest Moon rises about the same time that the sun sets, since both are close to the celestial equator and thus precisely opposite one another in the sky. In coming days, instead or rising about 50 minutes later each night, the Moon rises 10 or 20 minutes later. This "Harvest Moon effect" is more pronounced the further north you go.

You may well ask how the Harvest Moon can provide extra light at this time of year. I'll explain how it works in tomorrow's installment of SpaceWatch.