Posted on: Sunday, August 29, 2004

Shine on harvest moon, as days get shorter

By Mike Shanahan
Bishop Museum

Equinox

Autumn begins at 6:30 a.m. Sept. 22, Hawai'i time. At that moment, the sun will stand exactly over the Earth's equator.

It's the autumnal equinox.

Harvest moon

The full moon nearest to the fall equinox is called the harvest moon. This year, the harvest moon rises on Sept. 28.

Full moons always rise around sunset — as the sun goes down in the west, the full moon rises in the east. This is one reason why full moons are so memorable. They rise at sunset, are in the sky all night, and set around dawn.

In Honolulu, the harvest moon rises at 6:55 p.m. on Sept. 28. On average the moon rises about 50 minutes later each night. However, on Sept. 29 (the night after the harvest moon) the moon rises in Honolulu at 7:25 p.m. That's just 30 minutes later than on the night of the harvest moon. The next night, Sept. 30, the moon comes up right around 8 p.m., 35 minutes later. So for several days in a row, you have the bright light of a full (or nearly full) moon coming up right around dusk.

The September moon effect is more striking as you move north of the tropics. In Seattle, for instance, the harvest moon rises at 6:20 p.m. on Sept. 28 and a mere 20 minutes later on Sept. 29.

It Happens Every Year

This same situation occurs every year with the harvest moon. Because of the lineup of the sun, Earth and moon around the time of the fall equinox, the day-to-day difference in moonrises in the northern hemisphere is at its shortest in September, usually 30 minutes or less between one day and the next. (January has the longest day-to-day difference, with more than one hour between the time of moonrise from one day to the next.)

The big, bright moon rises at about the same time of the evening for several nights in a row. For farmers gathering in the harvest in September — especially in the days before artificial lighting — it was a real blessing to have a full moon popping over the eastern horizon to provide the extra light after sunset. Thus the term "harvest moon."

More on the harvest moon: science.nasa.gov/headlines/y2000/ast11sep_2.htm.

The days get shorter, the sun gets lower

During the late spring and summer, the sun has been high overhead in Hawai'i; but as we move into the fall, it sinks lower in the sky. On the first day of autumn in Honolulu, the sun will be only two-thirds of the way up from the horizon at noon. By the first day of winter, Dec. 21, the sun will be only 44 degrees above the horizon, or almost exactly halfway between the horizon and the top of the sky. (In early July, the sun is overhead at noon in Honolulu.)

Constellations

In September, we lose the Big Dipper. At the start of the month, you can still see the Big Dipper low in the north-northwest from dusk till about 9 p.m. By the end of the month, you can barely glimpse the seven famous stars of the Big Dipper around 7 p.m., and within a few minutes the Dipper begins to set.

The W-shaped constellation of Cassiopeia rises as the Big Dipper sets, so you always have one or the other of these famous constellations in the sky. Both point to the North Star. People generally use the two stars in the Big Dipper cup that are not attached to the handle as pointers to the North Star. The W of Cassiopeia can do the same: Just imagine that the point in the middle of the W is an arrow, pointing in the general direction of the North Star.

The bright star Hokule'a (Arcturus to the Greeks) is low in the west as it gets dark. It's the brightest thing in the western sky.

The Scorpion — known in Polynesia as Maui's Fishhook — is now in the southwestern sky as it gets dark.

While we lose the Big Dipper, we gain Orion. Orion will rise due east at about 2 a.m. at the start of September, but will be above the eastern horizon by midnight at the end of the month. Above Orion are the bright star Aldebaran and the lovely, tiny star cluster called the Pleiades.

Among the constellations that are missing in action in September: the Southern Cross, which is not visible from the Hawaiian Islands until early December.

Space flights

The Cassini probe (saturn.jpl.nasa.gov/operations/index.cfm) went into orbit around Saturn on July 1. Its next major event will be a close flyby of the giant moon Titan on Oct. 26.

Genesis, one of the most unusual NASA missions, returns to Earth Sept. 8.

Genesis was launched in the summer of 2001 to collect solar particles. As it heads toward the earth over Utah, parachutes will slow its descent. A helicopter will attempt to grab the space probe before it hits the ground.

This is the first time since the Apollo moon program that we've returned a space sample to Earth.

More on Genesis: www.nasa.gov/mission_pages/genesis/main/index.html.

Planets in September

Mercury: Mercury rises in the east at around 5:15 a.m. during the first three weeks of September. Unfortunately it doesn't get very far up before daylight washes out the planet.

Venus: Venus is brilliant in the morning sky all of September. It outshines all the other dots of light up there.

Look for Venus in the east; it rises at 3:15 a.m. at the start of the month and at 3:30 a.m. by the end of September. By daybreak Venus is about one-third of the way up in the eastern sky.

Look for a crescent moon next to Venus on the morning of Sept. 10.

Saturn: Look for Saturn in the morning sky. Saturn rises about 3 a.m. at the start of September and is a third of the way up in the sky at daybreak; by the end of the month the ringed planet rises just after 1 a.m. Saturn starts the month in a very nice gathering with Venus, though Venus is many, many times brighter. On Sept. 1, the two planets will be only 2 degrees apart.

Moon phases

Third quarter: Sept. 6
New: Sept. 14
First quarter: Sept. 21
Full: Sept. 28

• • •