A guide to the phases of the Moon, and why its appearance transforms night after night from crescent to gibbous and back again.

You are watching: Why do you think we see phases of the moon?


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The Moon may appear to readjust form, yet in truth the bideal surchallenge you watch and also the ‘moonlight’ that reaches Planet is actually sunlight showing off the lunar surchallenge. As the Moon orbits our planet, its varying place implies that the Sun lights up various regions, creating the illusion that the Moon is changing form over time.


The ideal way of gaining to understand also the lunar phases is to on a regular basis go out on a clear night when the Moon is in the sky and observe it. For more on this, check out our overview on how to observe the Moon.

On average 384,400km from Earth, it’s stunning to the naked eye and through binoculars or a tiny telescope, spectacular. It’s also a great taracquire to photograph. For more on this, read our overview on exactly how to photograph the Moon or our beginners’ overview to astrophotography.


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The phases of the Moon. The inner circle shows what the Moon looks prefer watched from over its north pole, while the external circle reflects the phase we view from Earth at that time. Credit: BBC Sky at Night Magazine
The Moon seems peaceful however it is hurtling eastward travelling at 3,682 km/h and also, considering that its nearly circular orlittle bit is tipped a mere 5° relative to Earth’s, it more or much less adheres to the ecliptic (the Sun’s noticeable path) throughout the sky.

You may have actually noticed that the Moon constantly keeps the same face turned in the direction of us. This is bereason it rotates when on its axis in specifically the exact same time it takes to orbit Earth – 27 days and also salso hours.


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Moon phases in the southerly hemisphere Luis Rojas M, Santiearlier, Chile, 13 October – 17 October 2018. Equipment: Canon EOS Rebel T6i, Explore Scientific 102mm ED refractor, iOptron iEQ30 Pro mount.
This synchronisation is referred to as tidal locking and is a result of Earth’s gravitational impact on the young Moon once it was developing.

During its elliptical journey around Planet, the Moon moves via ‘phases’, the term we use to explain how a lot of the lunar disc shows up illuminated as seen from Planet.

This elliptical orlittle bit, linked with the phases is additionally what leads to the appearance of a so-dubbed supermoon.



In truth, the Moon is constantly half lit, we simply don’t check out it that means. Whatever phase we’re seeing, the opposite phase is happening on the far side of the Moon.

And while we just ever watch one terminator (the name given to the dividing line between the light and dark parts of the lunar surface) sweeping right to left throughout the lunar disc at any kind of time, tbelow are actually 2 of them circumnavigating the Moon specifically 180° apart; the morning terminator (which ushers in the lunar day) and the evening terminator (which brings the night behind it).

So sorry, Pink Floyd, tbelow is no permanently dark side of the Moon.

The phases of the Moon

What many kind of people don’t realise (also though it’s completely logical), is that there’s also a partnership between the Moon’s phases and moonrise times.

New Moon


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New Moon (1% visibility) by Martin Marthadinata.
In this phase, our satellite is invisible. With the Sun and Moon on the same side of Earth, they climb together yet we cannot see the Moon as it’s concealed in the Sun’s glare. There’s not a lot to check out anyway, as its confront in the direction of us is entirely in shadow.

Waxing crescent


Credit: Sarah and also Simon Fisher.
Continuing its journey, the Moon’s western (right) edge becomes sunlit to produce a sliver-thin crescent. The morning terminator starts its creep of 15.5km/h from west to east.

First quarter


Credit: Paul Licorish
This one confoffers non-astronomers, bereason it clearly looks favor fifty percent a Moon, yet it’s called a quarter Moon. That’s bereason the terminator has actually completed a quarter (90°) of its 360° journey around the Moon.

By this logic a complete Moon must be called a fifty percent Moon, however that’s simply silly, right? In this phase, the Moon rises at noon and sets at midnight. Along the terminator, low-angled sunlight creates long shadows, throwing adjacent crater and mountains into sharp relief – perfect for lunar observations.

Waxing gibbous


Credit: Harvey Scoot.
In this phase the Moon is virtually totally illuminated. The daylight area shows up egg-shaped (gibbous) and also is increasing in size (waxing) everyday.

Full Moon


Full Moon at Perigee by Tom Howard
Halfmeans via the morning terminator’s journey, the Moon is on the opposite side of Earth from the Sun, via its near side fully illuminated and dazzling. Shadow-less, bleached and also flat-looking, it’s not great for monitoring – that’s a shame bereason in this phase it rises as the Sun sets, sets as the Sun rises and is visible all night long!

Waning gibbous


Credit: Sarah and Simon Fisher
The Moon’s western edge is being consumed by darkness as the evening terminator comes right into watch. The sunlit, egg-shaped location is diminishing (waning).


Last quarter


Credit: Andrew McNaught.
It’s salso days and also nine hrs considering that full Moon and also, currently 90° west of the Sun, just the Moon’s eastern (left) fifty percent is illuminated. At this phase it rises at midnight and also sets at noon and, favor the initially quarter phase, uses staggering views.

Waning crescent


Credit: Anadolu Agency / Getty Images
With just the eastern edge sunlit you’ll admire a beautiful ‘C-shaped’ crescent. Diminishing everyday (waning) it will soon disappear as the lunar cycle concludes and also the Moon retransforms to ‘new’. While the Moon might keep the exact same face turned to us, it stays a day-to-day altering delight to observe.


Lunar libration
Thanks to lunar libration we can observe slightly more than half of the Moon’s surchallenge. Credit: Pete Lawrence
Over the course of a lunar cycle, the Moon concurrently wobbles both latitudinally and also longitudinally. These oscillations are recognized as librations.

Libration in latitude – nodding – occurs because the Moon’s axis is slightly inclined family member to Earth’s, allowing us to peer just a small over its north and, later in the month, south poles.

Libration of longitude – shaking – occurs bereason the Moon travels fastest as soon as closest to Earth and slowest once farthest ameans.

Daily (diurnal) libration occurs because of our planet’s rotation.

We check out the Moon from slightly different perspectives once it rises and as soon as it sets, and this distinction in perspective manifests as a slight noticeable rotation in the satellite, first to the west and also then to the eastern.

See more: O Come O Come Emmanuel In Spanish, 'O Come, O Come, Emmanuel' In Spanish

The linked result of all the over implies that rather of seeing simply 50 per cent of the Moon, over time we actually get to check out around 59 per cent.


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Jane Eco-friendly is a fellow of the Royal Astronomical Society and author of the Haynes Astronomy Manual


Phases of the Moon by Tom Chitchild, Woking, Surrey, UK. Equipment: Sky Watcher Explorer 130p, Sony Cyberswarm W210
Moon Montage by Sarah & Simon Fisher, Bromsgrove, Worcestershire, UK. Equipment: Canon 600D, Mak 127mm scope
Moontage by Sarah & Simon Fisher, Bromsgrove, Worcestershire, UK. Equipment: Canon 600D, Mak 127mm scope
Moontage by Sarah & Simon Fisher, Bromsgrove, Worcestershire, UK. Equipment: Canon 600D, Mak 127mm scope

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