Slender Moons

Image Gallery A crescent moon and Venus at dawn over San Diego. JERRY SCHAD/PHOTO RESEARCHERS

Article Tools

Image Gallery

Print

E-mail

Comments

RSS

Making time for a new year.

The summer comes to an end and the autumn begins. It's not only time to go back to school, begin the harvest, and watch the migrations of birds. It's also time to Celebrate a Jewish New Year, look for slender Moons and Jupiter at its best, and stand alert for the threat of the most formidable type of hurricanes.

An Assortment of Astro-Sights

Around autumn equinox is not a good time to try to see the Moon as soon as possible after New Moon. This evening crescent is slung very low (in horizon haze and glow) off to the side of the setting Sun at this time of year (at least from our highly populated middle northern latitudes of Earth). The opposite is true of the Moon and planets in the dawns of August through October: their separation from the Sun is almost vertical, almost as steeply above where the Sun is going to rise as possible.

This latter fact is good news for anyone who wants to enjoy the conjunctions of Venus and Mars, and of Venus, Mercury, and the star Regulus this August and September. It also holds out a slight possibility of at least eastern U.S. observers seeing a wondrously slender lunar crescent low in the east only about 15 hours before New Moon at dawn on August 21st.

The opposite of New Moon is Full Moon. Full Moon is opposite (though rarely exactly opposite) the Sun in the sky. In other words, Full Moon rises around sunset, is highest around the middle of the night, and sets around sunrise. If the Moon is opposite the Sun in the sky, it will be pointing the same side of itself towards both Earth and Sun. And that means all of the Moon pointed at us will be sunlit: fully lit—Full.

Notice I said above that the Full Moon is rarely exactly opposite the Sun in our sky. If this were not true, there would be a lunar eclipse at every Full Moon. Most months, the Moon passes a little above (north of) or a little below (south of) Earth's shadow and isn't eclipsed. Other times all of it goes into Earth's "umbra," or dark central shadow, and there is a total lunar eclipse. Sometimes only part of the Moon enters the umbra and we see a partial eclipse. There is also what we call a "penumbral lunar eclipse."

The penumbra is the lighter, peripheral part of Earth's shadow. In a penumbral eclipse, the Moon doesn't go through any part of the umbra, only the penumbra. This penumbra is such a light shading that the Moon typically must enter half of its disk within the penumbra before we can see the slightest shadowing of the farthest-in part of the Moon. The penumbral lunar eclipse of August 7 is so slight it will be undetectable. But the one on September 6 will display a definite darkening on the Moon's upper or upper right edge—if you are far enough west in the U.S. for the Moon not to have set in the brightening dawn sky. Maximum eclipse occurs at 4:10 AM. PDT.

RSS | Comments | E-mail | Print