The first Jupiter image of the new observing season!
The Moon climbs through the dim autumnal constellations this week,
shedding her abundant light on an otherwise rather empty part of the
sky. She ends the week among the rising stars of the winter sky,
rising just before midnight. The Full Moon falls on the 19th at 7:13
am Eastern daylight Time. Since this is the Full Moon that occurs
closest to the Autumnal Equinox it is almost universally known in the
Northern Hemisphere as the Harvest Moon. It gets this name because it
happens to occur at the time of year when Boreal farmers are bringing
in their crops for the winter. It also happens to undergo a rather
interesting phenomenon that's associated with its apparent path around
the sky. You've probably noticed that over the past week the Moon
hasn't risen very far above the southern horizon. If you trace its
apparent path around the sky you'll also notice that this path will
intersect the eastern horizon at a shallow angle. Even though the Moon
moves about 13 degrees along the Ecliptic each day, if you measure its
elevation with respect to the eastern horizon for several days around
the date of Full Moon you'll find that its apparent elevation only
changes by some five to six degrees. This means that at the latitude
of Washington moonrise on successive nights around Full Moon takes
place just over a half hour later each night instead of the more
typical hour. This effect becomes more pronounced at higher latitudes.
In Scotland the difference is just 20 minutes, and in Iceland it's
less than 10 minutes! The net effect of all of this is that,
traditionally, farmers in northern Europe could rely on the
nearly-full Moon to extend the time they had to work in their fields
to bring in the harvest. Today's farmers probably don't need the extra
bit of light afforded by this phenomenon, but many are still keenly
aware of the folklore. This is actually one of two Harvest Moons that
occur each year; the other helps our friends "down under" bring in
their crops in March.
Full Moon isn't really the best time to explore Luna's varied
landscapes through the telescope, though. With sunlight falling almost
directly down on lunar features all sense of topography is lost, and
only differences in reflectivity mark her most prominent features. Her
surface also appears very bright, almost blindingly so, if you look at
her with optical aid. However, as a whole the Moon is a very poor
reflector of light. Her average surface albedo is about seven percent,
roughly the same value as a dark business suit!
The Autumnal Equinox occurs on the 22nd at 4:44 pm EDT. This is the
moment when the center of the Sun's disc reaches an ecliptic longitude
of 180 degrees, which also happens to correspond with the time it
crosses the celestial equator and enters the southern hemisphere sky.
You've probably noticed the rapid change in the length of daylight in
the past few weeks, especially in the time of sunset. This is the time
of year when length-of-day changes most rapidly, losing about two
minutes at sunrise and sunset each passing day. By the end of the week
daylight will be shorter than night for the next six months.
Venus and Saturn continue their race against the advancing Sun. You
can see the two objects close together during the first evenings of
the week, but Saturn is going to lose this race pretty quickly. The
two planets are within four degrees of each other on the 17th and
18th, but by week's end Venus will have left Saturn some 10 degrees in
her wake. The ringed planet will soon be lost in the glow of twilight,
but Venus will muster enough momentum to stay prominent in the sky
until early next year.
Jupiter is now very well-placed for viewing as morning twilight begins
to tint the eastern horizon. Old Jove is holding court in the middle
of the constellation of Gemini, surrounded by the bright stars of the
Great Winter Circle. I've managed to get up before the Sun on some of
the crisp cool mornings of late and have enjoyed seeing the giant
planet along with his glowing attendants. Winter nights won't seem so
long with these bright companions in the sky!
Mars hasn't quite cleared my roofline before dawn, but he is still
easy to spot as morning twilight gathers. The red planet is now
setting his sights on Regulus, the heart of Leo, the Lion. He'll pass
very close to the star in mid-October. Right now he doesn't offer much
for the telescope, but this spring we'll have a chance to watch him
brighten and grow in the eyepiece as he moves toward opposition.
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