Astro Bob: Metamorphosis at dusk — A New Moon revelation
This week, the crescent moon returns to the evening sky after a little alone time in its "chrysalis phase."
Tonight — Monday, Sept. 6 — at dusk, the moon will be new, its most rarely observed phase. This will occur at 7:51 p.m. Central Daylight Time, when it will pass almost directly between the sun and Earth. Since the moon appears in the same direction as our star it's impossible to see.
Don't even think about looking. It won't do you any good anyway. No sunlight falls on the side of the moon facing us at new phase. It's nighttime and dark across the entire near side. Only the back or far side of the moon — which we can't see from Earth — faces the sun at new moon. Can you guess what phase the moon is in from that side? If you answered full moon, you're correct! Near side and far side phases are always complementary. At new moon, the far side moon is full. When we see a full moon, astronauts orbiting above the lunar far side see a completely dark new moon.
During most new phases the moon isn't directly in line with the sun. That's certainly true Monday evening (September 7) when it's 4° to its north. The sun, moon and Earth only align in a neat, straight line twice a month. If that happens at new phase, the moon passes directly in front of the sun, and we get a total solar eclipse. If it occurs at full phase, we experience a total lunar eclipse.
Perfect or near-perfect alignments happen only occasionally because the moon's orbit is tilted about 5° to the plane of the Earth's orbit. Most of the time it passes up to 5° above or below the sun and no solar eclipse occurs. The next time the three bodies square up at new moon will be on Dec. 4 . Observers on board cruise ships in the Southern Ocean from the South Orkney Islands to the Antarctic ice cap will get the chance to see the new moon totally eclipse the sun.
The moon's average orbital speed is around 2,300 miles an hour or about a kilometer a second. The direction of its motion as seen in the sky is to the east — left in the northern hemisphere and right in the southern. After new phase, the moon gradually departs the sun, moving up and to the left for northerners. As it does, the angle it makes to the sun-Earth line increases much like the minute hand moving past the hour hand at the 12 noon position on a clock.
As the angle widens we first see the extreme edge of the moon in sunlight. Because the moon is a sphere, the edge is curved, assuming the shape of a crescent. Tuesday night's (Sept. 7) crescent will be very, very thin because the angle is so slight. But it increases nightly as the moon continues along its orbit, and the crescent thickens.
When the angle the moon makes with the sun reaches 90°, we see exactly one-half of the near side in sunlight. Since this occurs when the moon has covered 1/4 of its orbit, that lunar phase is called a first quarter moon.
While still a thin crescent, the moon will pass about 5° above the planet Mercury on Wednesday, September 8th low in the western sky at dusk. Although Mercury shines brightly at magnitude 0.0, sky glow from twilight will probably make it impossible to see without binoculars. Find a place with a wide-open view to the west as far down to the horizon as possible. Sweep slowly to the left of the brightest sunset glow to catch sight of the moon, then look close to the bottom of the binocular field of view for a single spark of light — Mercury.
The following night, Sept. 9, the moon thickens and brightens while at the same time it's higher in the sky and considerably easier to spot. It's also in conjunction 3.5° north (above) of Venus. Both are super-easy to see. Look between 30 minutes and an hour after sunset. They should make a very photogenic pair.
During the next week, Mercury gets a little farther from the sun, then circles back toward it and disappears in the solar glare. While Venus's apparent distance from the sun increases throughout the fall, the angle the planet makes to the western horizon bends toward the south. For northern hemisphere observers that means the planet remains low at dusk through mid-November. But Venus is so bright, it's always easy to see. Clear skies!
"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob .