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Now and then a reader asks why the Moon isn't always in our night sky. While the answer is obvious to astronomers, most people have lives outside astronomy, or more to the point, they have lives that are mostly inside buildings.
Modern humans have lost touch with the night sky compared to our ancestors, to the point that even some of the most intelligent people on the planet are too wrapped up in the day-to-day to know much about what's going on at night.
But grasping the phases of the Moon is easy. And as fears of inadequate knowledge wane, anyone can wax on to friends, family or colleagues with newfound knowledge of lunar comings and goings.
Coming up
First, some basics. The Moon rises because Earth won't stop rotating.
Each day, though not at the same time, the Moon comes up in the East and goes down in the West -- much like the Sun and other stars and for the same reason: Earth rotates, on its axis, toward the East, pulling celestial objects into view and then forcing them to slip away.
Let's visualize this: Put yourself on a merry-go-round, facing outward. With each revolution, a stationary friend who was afraid to ride comes into view at a certain point and then leaves your field of vision with a half-turn of the ride. For nitpicking readers, this analogy assumes you don't peek back through the legs of the horses while on the other side of our imaginary Earth.
This is not the only important motion in the Earth-Moon system, though.
The Moon makes an orbital trip around Earth once every 29.5 days. In the sky, this gradual movement is eastward, though it's not perceptible during any given observing session. It is, however, why the Moon rises later every night or day.
Going through phases
This monthly eastward creep is responsible for lunar phases.
At Full Moon, the swooners' favorite satellite is exactly opposite the Sun, with Earth in the middle (picture this as though you were looking down on the solar system from above). From our surface perspective at sunset, the Moon is rising and it reflects a full disk of sunlight directly to our eyes. The Moon arcs across the sky and sets at sunrise.
The night after it is full, the Moon rises later, typically by about 50 minutes. Just to confound you're understanding, this time frame varies with the seasons and with latitude, as SPACE.com's Night Sky columnist Joe Rao explained Friday. This year's fall Harvest Moon, which occurs this week, rises less than a half-hour later each night at mid-northern latitudes like San Francisco. In the far north, the Moon this week will actually rise earlier each night. [More about this].
After a few nights, the Moon may be coming up after you go to bed -- one reason you would not see it.
At last quarter -- a week after the full phase -- the Moon appears as a backward "D" and does not rise until midnight, remaining in the morning sky until Noon. In this phase, the positions of the Sun, Earth and Moon form a triangle when viewed from above. Sunlight still strikes an entire disk of the Moon, but we can only see the portion that's reflected toward Earth. Another half of the Moon -- shaped like a regular D -- still faces Earth, it just isn't illuminated.
Roughly a week later, the Moon has inched around in its orbit and is directly between Earth and the Sun. This is called New Moon, and it isn't visible. The New Moon rises and sets right along with the Sun, so no sunlight can be reflected off the side of the Moon facing us.
(During New Moon, an eclipse of the Sun can occur. But because the Moon's orbit is tilted slightly with respect to the orbit of Earth around the Sun, only when the paths come into certain alignment does an eclipse result.)
At first quarter -- roughly seven days after the new phase -- the Moon looks like a regular D. It rises around Noon and is then visible all afternoon and until about midnight, when it sets.
For the record, the Moon always shows the same face to Earth, whether we can see it or not. This is because in the exact time it takes the satellite to orbit around the Earth once -- the 29.5 days -- it also revolves one time on its axis [More about this].
Try this at home
If all these words just muddle things in your mind, try making your own solar system.
Use a large camping flashlight to act as the Sun in an otherwise unlit room. A table lamp will do if you're not the outdoorsy type. Place an apple on the table to represent Earth.
I recommend a golf ball as the model Moon -- it's highly reflective and has crater-like pits for added realism.
Move the golf ball counterclockwise around the apple and watch how sunlight shines on it. One circle represents a full lunar cycle -- not quite a month on the calendar. Inspect what part of the golf ball is lit from the perspective of a worm on the apple, and note whether it is daytime or nighttime wherever you'd like to imagine the worm to be.
Your model solar system should help illuminate celestial rhythms, revealing where the Moon really goes.
This article is part of SPACE.com's weekly Mystery Monday series.
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