Why does the full Moon look as big as the Sun?
The diameter of the Moon is only 3476 km (2160 miles), about the distance from London to Beirut. The Sun, diameter 1.39 million km (864 000 miles), is roughly 400 times as big. Yet when we look at a full Moon in the sky it seems to be about the same size as the Sun. The apparent sizes match so closely that occasionally the Moon’s disc shuts out the Sun’s light completely, causing a full eclipse.
What we see as a similarity in size is just an accident of time. The Moon, though much smaller, looks as big as the Sun because it is much closer. On average, the Sun is about 150 million km (93 million miles) distant, 389 times farther away from us than the Moon.
In the past, the Moon was much closer. At one time, 1200 million years ago, it was as close as 18 000 km (11 200 miles) and would have appeared about twenty times as big as it does today. Gradually, it is moving away, an effect of the Earth’s tides. Gravitational pull from the Moon causes our oceans 70 per cent of the Earth’s total area to bulge. If the Moon is over the Atlantic, the pull causes a bulge in that ocean and another in the Indian Ocean on the far side of the globe. These bulges are seen by us as tides, and their frictional effect, acting like brakes, slows down the Earth’s rotation, lengthening our day by about one second in 62 500 years.
This gradual slowing of the Earth allows the Moon to spin more quickly, inducing it to move away. The Earth’s rotation also has an effect on the tides, drawing the bulges eastwards. This great shift of water affects the Earth’s gravitational force, exerting a sideways tug on the Moon that also causes a gradual speed up in its orbit. In this process, the lunar orbit itself is growing bigger so that, even though the Moon is travelling faster, it will take longer to make its sweep around the Earth.
Calculations show that this process will go on for several million years, until eventually the Moon takes fifty-five days to complete its orbit, twice its current time of twenty-seven and a third days. That doesn’t mean that we will eventually see the far side of the Moon, because as the Moon takes longer to complete its orbit, the Earth’s spin will slow down, keeping the two bodies in step, like a well trained couple of ballroom dancers.
What we see as a similarity in size is just an accident of time. The Moon, though much smaller, looks as big as the Sun because it is much closer. On average, the Sun is about 150 million km (93 million miles) distant, 389 times farther away from us than the Moon.
In the past, the Moon was much closer. At one time, 1200 million years ago, it was as close as 18 000 km (11 200 miles) and would have appeared about twenty times as big as it does today. Gradually, it is moving away, an effect of the Earth’s tides. Gravitational pull from the Moon causes our oceans 70 per cent of the Earth’s total area to bulge. If the Moon is over the Atlantic, the pull causes a bulge in that ocean and another in the Indian Ocean on the far side of the globe. These bulges are seen by us as tides, and their frictional effect, acting like brakes, slows down the Earth’s rotation, lengthening our day by about one second in 62 500 years.
This gradual slowing of the Earth allows the Moon to spin more quickly, inducing it to move away. The Earth’s rotation also has an effect on the tides, drawing the bulges eastwards. This great shift of water affects the Earth’s gravitational force, exerting a sideways tug on the Moon that also causes a gradual speed up in its orbit. In this process, the lunar orbit itself is growing bigger so that, even though the Moon is travelling faster, it will take longer to make its sweep around the Earth.
Calculations show that this process will go on for several million years, until eventually the Moon takes fifty-five days to complete its orbit, twice its current time of twenty-seven and a third days. That doesn’t mean that we will eventually see the far side of the Moon, because as the Moon takes longer to complete its orbit, the Earth’s spin will slow down, keeping the two bodies in step, like a well trained couple of ballroom dancers.
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