The Moon is the most famous of all night sky objects, because it’s so large, bright, regularly visible and easy to observe. Orbiting Earth at 384 thousand kilometres, it takes light reflected off the lunar surface just over a second to reach us. The source of that light is, of course, the sun, from which light takes eight minutes to reach Earth. As is demonstrated by these travel times, the Moon is a hell of a lot closer than the Sun, and of course, the rest of the planets. It’s the fourteenth largest object in the Solar System, only four times smaller than Earth. Due to its close proximity and considerable size, the Moon is the easiest object in the solar system to observe in detail. Cool phenomena can even be seen with the naked eye.
It’s well known that the Moon is tidally locked to Earth, so we can only ever see one side. The phases, from a new Moon, to a crescent to gibbous to full moon and back again, are caused as the Moon orbits Earth. Imagine if someone walks between you and a lamp – they would be almost entirely illuminated to begin with, but as they get closer, you would see more and more shadow, until they are in silhouette. When they begin to walk away again, they would become more illuminated.
That’s massively oversimplifying, but it’s a good start. The Moon’s orbital path is elliptical, so it moves closer to and further away from Earth as it traces an oval in space. This elliptical path is also tilted by about five degrees, meaning it rarely passes completely between Earth and the Sun, or into Earth’s shadow.
All of these components come together to create various spectacles that can easily be viewed from Earth.
One of the most common lunar ‘events’ is a supermoon – this is when the Moon is at its closest point to Earth, whilst also appearing ‘full’. This makes it appear slightly larger than normal. The ‘full’ phase occurs when the Moon is on the opposite side of the Earth to the Sun (a new moon, when none of the Moon is illuminated from our viewpoint, is when the moon is in the opposite position). However, due to the inclination of its orbit, it rarely falls into the Earth’s shadow. Occasionally it does, but I’ll get into that a bit later.
Earthshine is an Earth-specific version of Planetshine, an effect that occurs on objects in close orbit with their parents. Light from the Earth is reflected back to us off the ‘dark side’ of the moon, making the dark side appear to glow slightly. This can occur during any lunar phase, but is more apparent during smaller crescents or a new moon. It can make the disc of a new moon entirely visible, even if just very faintly.
The most famous lunar spectacle is a solar eclipse, when the moon moves directly between the Earth and the Sun. They’re particularly rare, as the orbital plane of the Moon needs to move right in front of the Sun from our perspective, and the Moon has to be in the right place. We’re very lucky, as the Sun and the Moon are placed pretty much perfectly for us to get the coolest looking eclipses. The Sun is four hundred times further away than the Moon, but four hundred times larger. This means that the moon perfectly covers its surface, but leaves the surrounding area visible during a total eclipse. This allows us to see the Sun’s corona, but I’ll go into that more in another article. When the Moon is at its farthest point from Earth during a solar eclipse, a slightly different type will occur, known as a ‘ring of fire’ eclipse. The difference here is that the Moon doesn’t cover the entirety of the solar disc, and so a ring of flame is visible around the Moon.
A lunar eclipse occurs when the Earth is between the Moon and the Sun, essentially casting a shadow on the moon, which covers its entire face. They aren’t as cool as solar eclipses, but are more easily visible. Whereas the shadow of a solar eclipse is only visible from certain places, a lunar eclipse is visible everywhere the moon is at the time, meaning that half the surface of Earth can view it. They’re really cool events, and are visible to the naked eye with no more difficulty than the moon is. There are three types of Lunar eclipse, which I’ll do my best to explain.
The Earth casts two shadows, the penumbra, and the umbra. The penumbra is the widest, where light from the sun is only partially blocked by Earth. It occurs in positions behind the Earth where the solar disc is only partially blocked, be it one percent, or ninety-nine. Nonetheless, light is still there, albeit slightly less than normal. The umbra is the shadow that occurs in the area where the solar disc is completely blocked by the Earth. Here, there is no light from the sun at all.
When the Moon passes through the penumbra, a penumbral lunar eclipse will occur. Part, or all of the Moon’s surface will be dimmed slightly. Definitely the most unimpressive of the three.
When part of the Moon’s surface passes through the umbra, a partial lunar eclipse will occur. In this event, part of the Moon’s surface will become far darker than usual, if not completely dark, blending in with the night sky behind it. It looks as if some celestial beast has trotted up and taken a bite out of it.
A total lunar eclipse occurs when the Moon is completely inside Earth’s umbra. In these cases, it will turn red, receiving light only through Earth’s sunrises and sunsets, earning it the somewhat spookier name, a ‘Blood Moon’.
All of these events can be observed with the naked eye. However, use of binoculars or telescopes to observe the moon can reveal far more detail. Personally, I find that a full Moon is the worst for observation. Its brightness creates a lot of glare, and no craters are visible along the terminator (the ‘line’ between the illuminated and concealed sides). Crescent or Gibbous moons provide a better opportunity – the Sun casts shadows behind crater walls and mountains, making surface details normally difficult to spot more visible. My personal favourite crater is Copernicus – its size and depth make it very easy to spot, and it’s in shadow quite regularly.
I recommend taking a look at the Moon through a telescope or binoculars, and checking when you can next see an event like a supermoon or eclipse. If you get to take a look up close, google maps now has an interactive map of the Moon, which I’ve found incredibly helpful to name craters that I see.
The Moon also has ‘seas’, spots of darker land, composed of materials that reflect less light. They’re easily recognisable, and give the Moon its distinctive face. The most famous is the Sea of Tranquility, where the Apollo 11 mission landed. They were formed by basaltic lava drying up in crater basins, and are the Moon’s most famous surface features.
There are other types of ‘Moon’, that are distinguished only through the dates or times of year they occur upon. I’ll be covering this in another article about traditions and folklore about the night sky’s features.
The Moon is probably the best object in the sky if you’re just starting out, but it’s by no means boring. It has a lot of influence over our planet, and creates some of the most beautiful phenomena that can be seen from Earth.