Astronomy topic: Why are days so long on the Moon? 2

earth-moon tides

Time and tide wait for no man

JISHOU, HUNAN — China’s lunar probes, Chang’E and YuTu (Jade Rabbit), are preparing for Lunar day 3 of their mission, but they’ve been on the Moon since Dec. 15. Are they lazy, or what?

Considering a day on the Moon is almost two Earth weeks long, I’d say not.

Time for a quick astronomy lesson.

You know, I hope, that the Moon takes about 28 days to go around the Earth. This is where we get the English word “month” from (as in, “moonth,” the way they said it long ago).

Like the Earth, the Moon also rotates around its axis, but much more slowly. Earth takes about 24 hours for one complete spin, the Moon, about 28 Earth days from sunrise to sunrise.

Chang’E and YuTu use solar panels for power during the long lunar day. But during the lunar night, they hunker down, relying on small radioactive “batteries” to keep critical electronics warm and functioning.

Since there is only one lunar day each Earth month, the two probes have only been on the Moon for three lunar days. Each work shift is about 14 Earth days long, and they “sleep” for 14 days between shifts.

It is no coincidence that the lunar day-night cycle takes as long as its 28-day orbit around the Earth. This situation is called tidal locking, and it means that we have only been able to see one side of the Moon throughout human history.

synchronous moon orbit

Left: Moon as it behaves now. Right: Moon as it behaved long ago.

It was not always that way. When the Earth and Moon formed almost 4.6 billion years ago, the Earth spun faster around its axis and the Moon was closer than its current 384,000 km (240,000 mile) average distance. According to one estimate, 900 million years ago, the day-night cycle on Earth was only 18 hours long. Source

Large celestial bodies pull unevenly on one another, because the force of gravity falls off with distance. In other words, the Earth pulls harder on the near side of the Moon than on the far side. Over millions of years, that uneven pull slowed down the spin of the Moon until it spins only once for every orbit around the Earth.

Meanwhile, the Moon’s uneven pull helps cause the ocean tides on the Earth. (While it may be hard to believe or feel, the surface of the Earth rises and falls, too, but not as dramatically.) It also means the Earth’s rotation is also slowing down very gradually, about 0.0016 seconds every 100 years.

The Earth is about 10,000 81 times heavier than the Moon, so it will take much longer, but eventually the Earth will also slow its spinning to match the Moon’s orbital period. Fifty billion years from now, people living on one side of the Earth would always see the Moon high in the sky. People on the other side of the Earth would not. Oh, the days would be longer — 47 days (not hours!) from sunrise to sunrise.

And wait! That’s not all! The Earth and Moon are gradually moving away from each other, about 38 mm (1.5 inches) a year. Source In that far distant future 50 billion years from now, the Moon would appear about a third smaller than it does now, because it will be about 120,000 miles farther away. Source

Given that the Sun will enter its red giant phase in another 5 billion years, this dreary arrangement of Earth and Moon is a moot point. More than likely, both Earth and Moon, or what’s left of them, will be inside the Sun as it expands to about 320 million km (200 million miles) across.

Even if Earth and Moon escape being swallowed by the Sun, surface temperatures on the Earth will be above the boiling point of water, so it’s doubtful any people or animals will be around to witness the event.

So, enjoy your day! It could be a lot worse!

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2 thoughts on “Astronomy topic: Why are days so long on the Moon?

  1. Reply Marek May 19,2014 10:45 pm

    “The Earth is about 10,000 times heavier than the Moon, so it will take much longer, but eventually the Earth will also slow its spinning to match the Moon’s orbital period.”

    According to NASA website the Earth is only about 81.3 times more massive than the moon.

  2. Reply eljefe May 21,2014 9:23 pm

    You are correct. I have no idea why I said Earth is 10,000 times heavier than the Moon. From Universe Today:

    The mass of the Moon is 7.347 x 1022 kg. But the Earth is much more massive. The mass of the Earth is 5.97 x 1024 kg. This means that the mass of the Moon is only 1.2% of the mass of the Earth. You would need 81 objects with the mass of the Moon to match the mass of the Earth.

    I have corrected the post. Thanks for the catch!

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