{"id":54196,"date":"2024-01-24T10:11:55","date_gmt":"2024-01-24T10:11:55","guid":{"rendered":"http:\/\/d3fb2132-22ed-4f6c-aad3-083e7468bbbd"},"modified":"2024-01-24T12:32:31","modified_gmt":"2024-01-24T12:32:31","slug":"the-reason-we-always-see-the-same-side-of-the-moon-explained","status":"publish","type":"rss_feed","link":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/rss_feed\/the-reason-we-always-see-the-same-side-of-the-moon-explained\/","title":{"rendered":"The reason we always see the same side of the Moon explained"},"content":{"rendered":"

Why does the same side of the Moon always face Earth? <\/p>

By Kev Lochun\n <\/p>

Published: Wednesday, 24 January 2024 at 10:11 AM<\/p>

\n\n

From our perspective on Earth, we always see the same side of the Moon, and we know this because if we look at the Moon on a regular basis, its phases may change but we always see the same craters and other features.<\/p>

The distinctive pattern of bright highlands and dark lunar maria<\/a> on the Moon has been turned towards us for millennia, visible to every human who has ever stood on Earth.<\/p>

But why do we only see this one side of the Moon? We know that Earth spins about its axis, so why don\u2019t we get to see the full lunar surface as our Moon does the same?<\/p>

Full Moon over the Castle of Evoramonte by S\u00e9rgio Concei\u00e7\u00e3o, Evoramonte, Portugal, 6 May 2023Equipment: Canon EOS R mirrorless camera, Canon EF 100-400mm f\/4.5-5.6L IS II USM lens<\/figcaption><\/figure>
The Moon spins, so why don’t we see different sides of it?<\/strong><\/h2>
In simple terms, the Moon rotates once every 27.3 days, which is the same amount of time it takes for the Moon to orbit our planet. The result? On Earth we always see the same side of the Moon<\/p>
Although this is not strictly true, either. We do get to see slightly more of the Moon than half of the Moon’s surface at due to a wobbling effect known as lunar libration<\/a>.<\/p>
$\"Thanks$
We always see the same side of the Moon, although thanks to lunar libration we can observe slightly more than half of the Moon\u2019s surface. Credit: Pete Lawrence<\/figcaption><\/figure>
If the Moon were to spin faster or slower than once per orbit we would see all of it.<\/p>
In the proper astronomical parlance, we say that the Moon is \u2018tidally locked\u2019 to Earth<\/p>
You may also come across the expression \u2018synchronous rotation\u2019, which means the same thing.<\/p>
**How the Moon became tidally locked<\/strong><\/h2>** $\"A$
A diagram showing the formation of the Moon out of collision debris.<\/figcaption><\/figure>
When the Moon formed<\/a> some 4.5 billion years ago, it was spinning much more rapidly than it is today.<\/p>
Earth\u2019s gravity causes a rocky tidal bulge in our companion, which means it is lemon-shaped rather than a neat sphere, with a pinched end facing our planet.<\/p>
Back in the Moon\u2019s fast-spinning early history, the location of that bulge kept changing, shifting across the surface much in the same manner as our ocean tides.<\/p>
This effectively acted as a brake, gradually slowing our companion\u2019s spin speed until it fell into equilibrium with its orbital period.<\/p>
At this point the hemisphere facing us became locked in place.<\/p>
**Why the Moon’s appearance changes<\/strong><\/h2>** $\"The$
The Moon may always keep the same face turned to us, but that face keeps changing. Image Credit: iStock<\/figcaption><\/figure>
Though the Moon always keeps that same side towards us, even a cursory glance will show you that it is not consistently illuminated from one night to the next.<\/p>
What you are seeing here is the changing phase of the Moon. By phase, we simply mean the proportion of sunlit Moon visible from Earth.<\/p>
The essential point to remember is that although only a fraction of the Moon may be lit from our vantage point, a full 50% of the Moon is lit at any one time. We just can\u2019t always see it.<\/p>
$\"The$
The far side of the Moon, as seen by the Lunar Reconnaissance Orbiter. Credit: Credit: NASA\/Goddard\/Arizona State University<\/figcaption><\/figure>
This is why astronomers don’t like to refer to the side of the Moon facing away from us as the ‘dark side’, because this is not always true. Instead, we refer to the far side of the Moon<\/a>.<\/p>
The cycle of lunar phases (also known as a lunation) runs from new Moon to full Moon<\/a> and back again, passing through crescent Moon<\/a>, quarter Moon<\/a> and gibbous Moon<\/a> along the way, and takes 29.5 days to complete.<\/p>
***For more on this, read our guide to the*** ***phases of the Moon<\/a> and find out when the next full Moon is appearing.<\/em><\/strong><\/p>***
**The lunar cycle\/orbit discrepancy<\/strong><\/h2>** $\"Illustration$
Illustration showing how the phases of the Moon are caused. Credit: BBC Sky at Night Magazine<\/figcaption><\/figure>
Although it takes the Moon 29.5 days to complete a lunar cycle (a period known as the synodic month), it only takes 27.3 days to complete one orbit of our planet (a sidereal month).<\/p>
This discrepancy arises from one lunar cycle being defined as the time it takes for the Moon to return to the same phase as seen by an observer on Earth.<\/p>
Because Earth itself is moving, hurtling through space on its own orbit around the Sun, it takes the Moon that little bit longer to catch up than complete an orbit of its own.<\/p>
You may also wonder why, given the Moon sits in the middle of a line with Earth and the Sun at the point of new Moon, a solar eclipse<\/a> is such a rare event.<\/p>
And likewise, why we don\u2019t experience guaranteed lunar eclipses at the time of full Moon. It\u2019s because the Moon\u2019s orbit around Earth is tilted by around 5\u00b0 with respect to Earth\u2019s orbit around the Sun.<\/p>
What happens, in most instances, is a near miss.<\/p> <\/body><\/html>\n
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