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Skywatching

When is the moon full?

If you have never watched the full moon rising above the hills on the other side of a lake, you really need to do something about that.

It is beautiful and special. An added mystique comes from knowing we are doing something even our remotest ancestors must have done.

This may be one of the reasons the phases of the moon are listed in most of our diaries and calendars. However, if so why are the dates of the lunar phases sometimes wrong, usually out by a day?

Our ancestors must have taken note of the phases of the moon, although they did not know what was going on. They reckoned the passage of time using the moon. That is where the word "month" came from.

Today, we do understand what causes the lunar phases. They arise from the direction we are looking at the moon compared with the direction of the sunlight illuminating it. We can only see the moon because of the sun lighting it up.

In fact, unless sunlight is being reflected from the Earth onto the unlit part of the moon, a phenomenon known as the "Old Moon in the New Moon's arms,” we only see the sunlit portion.

Imagine you are sitting way out in space, way above the Earth's North Pole. You will see the moon circling the Earth like a ball on a string. The half facing the sun is lit up and the other half dark and invisible.

When the moon lies between the sun and us, the unlit side is facing us. In addition, when the moon is in that position, it is close to the sun in the sky and it is totally lost in the glare.

We refer to the moon at this time as being new.

The path of the moon around the Earth is in the eastward direction. So it moves leftward from the sun until a short time after being new, when it appears as a thin crescent in the western sky after sunset.

What we are seeing is a small sliver of the sunlit half. Many amateur astronomers take on the challenge of seeing the youngest moon possible, as a thin thread of crescent in the sky just after the sun has gone.

From that point, every day the moon will be about 12 degrees further east and rise about 50 minutes later. During this part of the cycle, the waxing moon is lit from the right, so it looks like a "D".

Around a week after the New Moon, the disc will appear exactly half lit, exactly like a "D". The moon is now at First Quarter.

After about another week, the moon will be at the opposite side of its orbit from when it was New. We will be between it and the sun and it will be lit from over our shoulders. We are then facing the fully lit side: the Full Moon.

It will continue on its orbit, over following days we will se an increasing fraction of the unlit part of the moon appearing on the right hand side. The moon is now waning and looks rather like a letter "C".

A week or so later, we will see the disc half lit, except this time lit from the right hand side. We are now at Last Quarter. From then on narrower, until the moon is again New. A simple guide is DOC: D-waxing, O-Full and C-waning.

The times and dates of the phases of the moon can be calculated accurately. We usually get this information in Universal Time (which used to be called Greenwich Mean Time). For example, on Jan. 21 2019, the moon was full at 05:16 UT.

Unfortunately, this is 12:16 a.m. EST on the 21st in Ottawa, but 09:16 p.m. on the 20th in Vancouver.

In general, most of the calendars we buy are printed in Europe or in eastern North America, with the dates of the phases of the moon given for those locations. Those of us living elsewhere have to live with the discrepancy.

This is not really a problem. We can get the exact times from the Web, or from the Observer's Handbook of the Royal Astronomical Society of Canada.

  • Jupiter and Saturn lie low in the southwest after dark.
  • The moon will be new on the 28th (in Eastern Canada) and the 27th (in the West).
  • Mars lies very low in the east before dawn.


More Skywatching articles

About the Author

Ken Tapping is an astronomer born in the U.K. He has been with the National Research Council since 1975 and moved to the Okanagan in 1990.  

He plays guitar with a couple of local jazz bands and has written weekly astronomy articles since 1992. 

Tapping has a doctorate from the University of Utrecht in The Netherlands.

[email protected]



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The views expressed are strictly those of the author and not necessarily those of Castanet. Castanet does not warrant the contents.

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