Category: Moon

Everyone knows that the Moon goes through phases, but let’s talk about why it does. It comes down to illumination, which in this case, all originates from our nearby star.

Our Moon orbits around our planet, and this Earth-Moon system orbits around the Sun.

Even though we only see light on part of the Moon, from the perspective of the Sun, half of it is always illuminated.

Stuck here on Earth, we see the Moon in various phases of illumination as it completes a 27.3 day orbit around the Earth.

As The Moon travels around us we see it pass through its phases. It goes from New Moon, to Full Moon and back to new Moon again.

Crescent Moons are when it’s less than half illuminated, and gibbous when it’s more than half.

“Waxing” means that the Moon becomes more illuminated night-by-night, and the term “waning” means that it’s getting less illuminated each night.

• New Moon – When the illuminated side of the Moon is away from the Earth. The Moon and the Sun are lined up on the same side of the Earth, so we can only see the shadowed side. This is also the time that you can experience solar eclipses, when the Moon passes directly in front of the Sun and casts a shadow onto the surface of the Earth. During a new moon, we can also see the reflected light from the Earth, since no sunlight is falling on the Moon – this is known as earthshine.
• Crescent – The crescent moon is the first sliver of the Moon that we can see. From the northern hemisphere, the crescent moon has the illuminated edge of the Moon on the right. This situation is reversed for the southern hemisphere.
• First Quarter – Although it’s called a quarter moon, we actually see this phase when the Moon is half illuminated. This means that the Sun and the Moon make a 90-degree angle compared to the Earth.
• Waxing Gibbous – This phase of the Moon occurs when the Moon is more illuminated that half, but it’s not yet a full Moon.
• Full Moon – This is the phase when the Moon is brightest in the sky. From our perspective here on Earth, the Moon is fully illuminated by the light of the Sun. This is also the time of the lunar month when you can see lunar eclipses – these occur when the Moon passes through the shadow of the Earth.
• Waning Gibbous – In this lunar phase, the Moon is less than fully illuminated, but more than half.
• Last Quarter – At this point of the lunar cycle, the Moon has reached half illumination. Now it’s the left-hand side of the Moon that’s illuminated, and the right-hand side in darkness (from a northern hemisphere perspective).
• Crescent – This is the final sliver of illuminated moon we can see before the Moon goes into darkness again.

If you ever get the chance to travel to the other hemisphere, you’ll immediately notice how unfamiliar the Moon behaves – it’s upside down.

If you live in the Northern Hemisphere, after a New Moon the crescent begins on the right-side. But if you’re in the Southern Hemisphere, it’s reversed, with the illumination starting on the left side.

Weird.

The alignment of the Sun, Earth and Moon can lead to some fantastic astronomical events.

One event occurs when the Moon is full, and it passes through the Earth’s shadow. Or as you probably know it, a lunar eclipse. This causes the Moon to grow dark and then turn an eerie red color.

When the Moon is new, it can pass in between the Earth and the Sun, casting its shadow down on our planet. As you know, a solar eclipse.

You’d think we would see a solar and lunar eclipse every month, but we don’t because the Moon’s orbit is inclined relative to the Sun.

Most months, the Moon is either above or below the Sun in the sky, so they just don’t line up perfectly.

One more thing, you might not know that Venus also goes through phases. When the planet is on the other side of the Sun from us, we see it as a nearly complete disk. But when Venus is on our side, just about to pass into the glow of the Sun, it’s a thin crescent, just like how we see the Moon.

I hope this gives you a better understanding of why the Moon goes through its phases every month, and the interesting relationship between the Earth, the Sun, and the Moon.

The night sky just wouldn’t feel right without the Moon. Where did our our friendly, familiar satellite come from?

Scientists and philosophers have been wondering about this for centuries.

Once Copernicus gave us our current model of the Solar System, with the Earth as just another planet and the Sun at the centre of the Solar System, this gave us a new way of looking at the Moon.

The first modern idea about the formation of the Moon was called the fission theory, and it came from George Darwin, the son of Charles Darwin.

He reasoned the Moon must have broken away from our planet, when the Earth was still a rapidly rotating ball of molten rock.

His theory lasted from the 1800s right up until the space age.

Another idea is that the Earth captured the Moon after its formation.

Usually, these kinds of gravitational interactions don’t go well.

Models predict that either the Moon would collide with the Earth, or get flung out into a different orbit.

It’s possible that the early Earth’s atmosphere was much larger and thicker, and acted like a brake, modifying the Moon’s trajectory into a stable orbit around the Earth.

Or the Earth and Moon formed together in their current positions as a binary object, with Earth taking most of the mass and the Moon forming from the leftovers.

The most widely accepted theory is that the Moon was formed when a Mars-sized object slammed into the Earth, billions of years ago.

This collision turned the newly formed Earth into a molten ball of rock again, and ejected material into orbit.

Most of the material crashed back into the Earth, but some collected together from mutual gravity to form the Moon we have today.

This theory was first conceived in 1946 by Reginald Aldworth Daly from Harvard University. He challenged Darwin’s theory, calculating that just a piece of Earth breaking off couldn’t actually allow the Moon to get to its current position. He suggested an impact could do the trick though.

This idea wasn’t given much thought until a 1974 paper by Dr. William K. Hartmann and Dr. Donald R. Davis was published in the Journal Icarus. They suggested that the early Solar System was still filled with leftover moon-sized objects which were colliding with the planets.

The impact theory explained many of the challenges about the formation of the Moon. For example, one question was: why do the Earth and Moon have very different-sized cores.

After an impact from a Mars-sized planet, the lighter outer layers of the Earth would have been ejected into orbit and coalesced into the Moon, while the denser elements collected back together into the Earth.

It also helps explain how the Moon is on an inclined plane to the Earth. If the Earth and Moon formed together, they’d be perfectly lined up with the Sun.

But an impactor could come from any direction and carve out a moon. One surprising idea is that the impact actually created two moons for the Earth.

The second, smaller object would have been unstable and eventually slammed into the far side of the Moon, explaining why the surface on the far side of the Moon is so different from the near side.

Even though we don’t know for sure how the Moon formed, the giant impact theory holds the most promise, and you can bet that scientists are continuing to look for clues to tell us more.