Image: Nine Planets

At sunset tonight, look to the horizon. The harvest moon--the most unique of all full moons each year--will hang low and large in the sky. What makes this moon so special is the time it rises. Throughout the year, moonrise usually falls 50 minutes later with each passing day. But near the autumnal equinox--September 22 this year--the day-to-day difference drops to 30 minutes. This September minimum is caused by the small angle that its ecliptic orbit makes with the eastern horizon in early autumn, which in turn makes the moon seem larger than when it sits high in the sky.

It is because this large moon appears so near sunset and gives off extra light that it is named for the harvest. It affords farmers longer working hours before autumn begins. And this year it should be especially luminous, thanks to recent wildfires in North America and dust storms in Africa. The aerosols these events kick up into the atmosphere will cast the harvest moon in a warm pink or orange glow.