6. Luna ticks. Luna tocks. What’s a month if the Moon’s a clock?

Background image by Peg via Wikimedia Commons.

The Moon is our heavenly companion.  In terms of significance, it is second only to the Sun.  It doesn’t blind us with its intensity.  It entices us, revealing a little more or a little less of itself each time we care to look.

Sometimes it disappears altogether.  This lasts for a couple of days and then we see a thin crescent.  This was traditionally referred to as a new moon but present-day astronomers use that term for the earlier missing phase.  Then, the illuminated portion of the Moon steadily waxes, or increases.  When it appears half-full, it is called a first quarter moon because a quarter of the cycle has been completed.  Then, when it is between half-full and full, it is called a waxing gibbous.  After the full moon are the waning, or decreasing, phases, namely waning gibbous, third quarter, waning crescent, and back to a new moon.

What we are actually seeing is the portion of the near side (Earth-facing side) of the Moon which is lit up by the Sun.  Just like the Earth, the Moon has a bright side (daytime side) and a dark side (nighttime side).  When the Earth lies between the Sun and the Moon, the near side is the same as the bright side and so the Moon looks full.  When the Moon lies between the Sun and the Earth, the near side of the Moon is dark since the Sun is then shining on the far side.  This is what we call a new moon.

Lunar phases in the northern hemisphere.  1. New Moon; 2. Waxing Crescent; 3. First Quarter; 4. Waxing Gibbous; 5. Full Moon; 6. Waning Gibbous; 7. Third Quarter; 8. Waning Crescent; 9. New Moon.  Objects and distances are not to scale.  The Sun is not shown but its direction is indicated by the yellow rays.  By Orion 8 via Wikimedia Commons.

One cycle of lunar phases is, on average, 29.5 days long.  This gave us the month, the second oldest unit of time after the day.  The fact that it is about as long as a woman’s menstrual cycle made it seem even more significant than it would have otherwise been.  To ancient peoples, this was yet another example of a mysterious connection between us and the heavens.

Astronomers refer to one cycle of lunar phases as a synodic month.

Is this the same as one orbit of the Moon around the Earth?  No.  That would be a sidereal month which is around 27.3 days long.

You can measure the length of a sidereal month the same way you can measure the length of a sidereal day.  Go out one night and see where the Moon is (if it’s up) in relation to some bright stars.  If you go out the next night, you will see that the Moon has drifted a bit.  From night to night, it will steadily drift against the background constellations.  It will take around 27.3 days to get back to the original position.

A synodic month is longer than a sidereal month for the same reason that a solar day is longer than a sidereal day.  During the time it takes the Moon to orbit the Earth, the Earth has moved in its orbit around the Sun.  

You can see this in the figure above if you pay close attention.  The first and last diagrams of the Earth-Moon system depict a new moon.  On the first diagram, the Moon lies above and slightly to the right of the Earth.  If the Moon’s orbit were a clock, the Moon would be at around 12:02 on the first diagram.  One sidereal month later, it will get back to the same clock position.  But after one synodic month, the Moon has to be located above and slight to the left of the Earth (or at around 11:58 on a clock) as shown on the last diagram.  The difference between the sidereal month and the synodic month is shaded in green on the last diagram.

Just as our first clocks were based on the cycle of day and night, our first calendars were based on lunar cycles.  Even today, some of the world’s calendars are purely lunar. But over time, another natural cycle became more and more important to us.  Some societies made room in their calendar for the Moon and this other cycle.  And some completely abandoned the Moon as a timekeeper.  

This other cycle is the cycle of seasons.  It is the subject of my next post.


Author: Bhasker Moorthy

I'm a Professor of Astronomy at Harper College in Palatine, IL, USA. My email address is bmoorthy [at] harpercollege.edu.

5 thoughts on “6. Luna ticks. Luna tocks. What’s a month if the Moon’s a clock?”

  1. Still like the idea of using a lunar calendar…. having 13 months instead of 12 and having each day of the week correspond to the same day…. Sundays will always be on the 1st, 8th, 15th, and 22nd…. adding just one extra day like a leap year each year and calling it day out of time or zero day to begin the cycle appears to be more practical than our current calendar.


  2. I like how there are connections between the sidereal day and a synodic month. Our perception of days and times are not exactly as they seem, for they are actually slightly different based on the moon and sun. A day is not truly a “day” and a month is not truly a “month”. Interesting how different cycles are more important and prevalent in our current society both with science and in popular culture. Good post.


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