Question of the Day
Does the Moon really shrink and grow every day?
Byeol-maru stopped far out in space where Earth and the Moon could be seen together. Outside the window, the Moon was slowly circling the Earth. One side of it glowed brightly, and the other side was dark.
“Byeoli, the Moon is only half lit.”
“Right, the Moon is always lit on one half by the Sun. It isn’t shrinking or growing. The Moon only looks different from Earth because, as it circles us, the angle from which we see its bright side keeps changing.”
“The angle changes?”
“When the Moon is on the same side as the Sun, we see its dark side, so we can’t see it at all. A bit later, we see a thin sliver of the bright side, a crescent. When it’s at a right angle to the Sun, we see a half moon, and when it’s on the opposite side from the Sun, we see the whole bright side, a full moon.”
“So doesn’t Earth’s shadow cover the Moon and change its shape?”
“That’s a very common mix-up. When Earth’s shadow covers the Moon, it’s called a lunar eclipse, and that only happens a few times a year.”
“Does the Moon have day and night too?”
“Yes. As it goes around once, one side has about two weeks of daytime and about two weeks of night. A day on the Moon lasts as long as two weeks on Earth.”
“How long does one trip around take?”
“The Moon circles the Earth in about 27 days. But during that time the Earth also moves around the Sun, so it takes about two more days for the same shape to come back. That’s why the cycle of phases takes about 29.5 days, roughly a month.”
“I heard Korea’s old calendar was made by following the Moon’s shape.”
“That’s right, the lunar calendar. If you know the lunar date, you can guess the Moon’s shape. Around the 1st you can’t see the Moon at all. On the 2nd or 3rd, a thin crescent appears in the western sky just after sunset. Around the 7th or 8th it’s a half moon bright on the right side, around the 15th a full moon, and around the 22nd or 23rd a half moon bright on the left side. On the 27th or 28th, a very thin moon rises in the eastern sky before dawn.”
“Can we only see the Moon at night?”
“No, you can see it in the daytime too. On days near a half moon or a full moon, you can spot a white Moon in the blue daytime sky.”
“Does the Moon rise at the same time every day?”
“No, it rises about 50 minutes later than the day before. If you saw it at 7 in the evening yesterday, you’ll find it around 7:50 today.”
“And here’s another thing. People living in the Southern Hemisphere, like in Australia, see the Moon upside down compared to us on the same day. It’s the same Moon, but it looks different depending on where you stand.”
Hyeongseop gazed at the Moon outside the window for a long time. The Moon he thought grew and shrank had never changed its shape at all. What had changed was the place he was looking from.
Ball and Lamp Experiment
In a dark room, turn on a single lamp and place it to one side. The lamp is the Sun, and your head is the Earth. Hold a ball, like a tennis ball or a mandarin, at arm’s length and imagine it’s the Moon. Slowly turn around in place once, and watch how the bright part of the ball changes. When you face the lamp, turn away from it, or look sideways, the ball will look like a crescent, a full moon, or a half moon.
“The Moon is always half lit by the Sun, and as it circles the Earth we simply see its bright side from different angles. A full cycle of phases takes about 29.5 days, roughly a month.”
How Much Did You Get Right? (True/False Mixed In)
- (T/F) The Moon actually shrinks and grows in size every day.
- How much of the Moon is lit by the Sun at any time?
- (T/F) The Moon’s shape changes because Earth’s shadow covers it.
- What do we call the Moon around the 15th of the lunar month?
- About how many minutes later does the Moon rise each day?
(Answers: 1. False, what changes is the angle we see it from, not its shape. 2. Half. 3. False, when Earth’s shadow covers the Moon it’s a lunar eclipse. 4. The full moon. 5. About 50 minutes.)
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