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The Sun and the Moon

Solar Eclipse. Image credit: NASA

Solar Eclipse. Image credit: NASA


The Sun and the Moon are the two objects in the Solar System that influence Earth the most. Let’s take a look at all the different was we experience these two objects, how they’re similar, and how they’re mostly different.

The Size of the Sun and the Moon

In absolute terms, the Sun and the Moon couldn’t be more different in size. The Sun measures 1.4 million km across, while the Moon is a mere 3,474 km across. In other words, the Sun is roughly 400 times larger than the Moon. But the Sun also happens to be 400 times further away than the Moon, and this has created an amazing coincidence.

From our perspective, the Sun and the Moon look almost exactly the same size. This is why we can have solar eclipses, where the Moon passes in front of the Sun, just barely obscuring it from our view.

And this is just a coincidence. The gravitational interaction between the Moon and the Earth (the tides) are causing the Moon to slowly drift away from the Earth at a rate of 3.8 centimeters per year. In the ancient past, the Moon would have looked much larger than the Sun. And in the far future, the Moon will look much smaller. It’s just a happy coincidence that they look the same size from our perspective.

Gravity from the Sun and the Moon

Once again, the Sun is much larger and has a tremendous amount of mass. The mass of the Sun is about 27 million times more than the mass of the Moon. It’s this gravitational interaction that gives the Earth its orbit around the Sun, and the tiny pull of the Moon just causes the Earth to wobble a bit in its movements.

When the Sun and the Moon are pulling on the Earth from the same direction, their gravity adds up, and we get the largest spring tides. And then, when they’re on opposite sides of the Earth, their forces cancel out somewhat, and we get neap tides.

Light from the Sun and the Moon

This is a bit of a trick, since the Sun is the only object in the Solar System actually giving out light. With its enormous mass, the Sun is able to fuse hydrogen into helium at its core, generating heat and light. This light shines in the Solar System, and bounces off the Moon so we can see it in the sky.

Astronomers measure brightness using a measurement called magnitude. The star Vega was considered 0 magnitude, and the faintest star you can see with the unaided eye is about 6.5 magnitude. Venus can get as bright as -3.7, the full Moon is -12.6, and the Sun is -26.73. These numbers sound similar, but it’s a logarithmic scale, where each value is twice the amount of the previous one. 1 is twice as bright as 2, etc.

So the Sun is actually 450,000 times brighter than the Moon. From our perspective.

Composition of the Sun and the Moon

Now here’s where the Sun and the Moon differ. The Sun is almost entirely composed of hydrogen and helium. The Moon, on the other hand, was formed when a Mars-sized object crashed into the Earth billions of years ago. Lighter material from the collision collected into an object in orbit – the Moon. The Moon’s crust is primarily oxygen, silicon, magnesium, iron, calcium, and aluminium. Astronomers think the core is metallic iron with small amounts of sulfur and nickel. And it’s at least partly molten.

Here’s an article about the distance from the Earth to the Sun, and here’s a view of the Earth and the Moon, seen from Mars.

Have you ever seen that picture of the Moon and the Sun “from the North Pole”, where the Moon looks huge? It’s actually a hoax, here’s more information from Astronomy Picture of the Day.

References:
NASA SOHO
NASA Starchild: Earth’s Natural Satellites
NASA Eclipse: Measuring the Moon’s Distance
NASA: Stellar Magnitude Scale

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