Astronomy/Moon Phases


Why do we have moon phases?

Moon phases
Moon phases  

Actually, the real question here is 'HOW do Moon phases occur?' since in science 'why' questions are not so important.

Attached is a diagram which will help explain the origin of lunar phases, from an article I wrote in the 1970s. The diagram includes the times at which an observer at a location on Earth will typically observe the approximate phase.

Note from the diagram how the Sun's rays are directed toward Earth - at the approximate center of the Moon's orbit. When the Moon is 'New' it will be directly in between the Sun's rays and the Earth.

As the Moon's orbit carries it in the direction of the arrow, only one half of the surface, as seen from the Earth- will recieve the Sun's rays. This is called 'first quarter' and appears as if the Moon is half lit. Now, in between the 'new' position and the 1st quarter position on the diagram you will see the 'crescent' phase, which means that the Sun's rays only cover one eighth or less the Moon's surface as seen from Earth.

In the direction the Moon is going - toward the first quarter, we say that the crescent is 'waxing' because it is getting larger toward the first quarter.

As the Moon moves from the first quarter position, it acuires an 'egg shape' - indicating the gibbous phase. We say it's 'waxing gibbous, because it's always increasing in size for this part of the orbit.

On the exact opposite side of the Moon's initial position in orbit (at New Moon) is the Full Moon. In this position, as you can see- the full face of the Moon facing the Earth is also facing the Sun's rays. Thus, we see what appears to be a fully lit Moon (though the opposite side of the Moon to our sight remains dark, since it's away from the Sun).

Next - after the Full Moon, the fraction of the Moon's sunlit surface facing the Earth begins to decrease again. So, we observe a 'waning gibbous' Moon between the Full Moon and the Last Quarter. Again, this appears as an 'egg-shaped' form or shape, less than full but more than a half.

At last quarter, you observe the same phase you did at first quarter, except in reverse- since you're on the opposite side of the lunar orbit.

In between the last quarter and the New Moon is the crescent phase again, though this is now a 'waning crescent' because it's getting smaller all the time as the Moon approaches the 'New' phase.

Finally, the Moon returns to its original position in orbit between the Sun and the Earth and we have 'New Moon' again..

The total time from the New Moon to the next New Moon is called a synodic period and is about 29.5 days. This means there are roughly four weeks between like phases: new moon to New Moon, crescent to crescent or Full Moon to Full Moon.


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Philip Stahl


I have more than forty years of experience in Astronomy, specifically solar and space physics. My specialties include the physics of solar flares, sunspots, including their effects on Earth and statistics pertaining to sunspot morphology and flare geo-effectiveness.


Astronomy: Worked at university observatory in college, doing astrographic measurements. Developed first ever astronomy curriculum for secondary schools in Caribbean. Gave workshops in astrophysics and astronomical measurements at Harry Bayley Observatory, Barbados. M.Phil. degree in Physics/Solar Physics and more than twenty years as researcher with discovery of SID flares. Developed of first ever consistent magnetic arcade model for solar flares incorporating energy dissipation and accumulation. Develop first ever loop solar flare model using double layers and incorporating cavity resonators.

American Astronomical Society (Solar Physics and Dynamical Astronomy divisions), American Mathematical Society, American Geophysical Union.

Solar Physics (journal), The Journal of the Royal Astronomical Society of Canada, The Proceedings of the Meudon Solar Flare Workshop (1986), The Proceedings of the Caribbean Physics Conference (1985). Books: 'Selected Analyses in Solar Flare Plasma Dynamics', 'Physics Notes for Advanced Level'. 'Astronomy and Astrophysics: Notes, Problems and Solutions'.

B.A. Astronomy, M. Phil. Physics

Awards and Honors
American Astronomical Society Studentship Award (1984), Barbados Government Award for Solar Research (1980), Barbados Astronomical Society Award for Service as Journal Editor (1977-91)

Past/Present Clients
Caribbean Examinations Council, Barbados Astronomical Society, Trinidad & Tobago Astronomical Society.

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