“Strawberry Moon”

I’m a little late getting this up, but these two diagrams illustrate what you see at latitude 40 N on the summer solstice, when there is a full moon (so-called “strawberry moon – borrowed from folklore). This doesn’t occur often, but an almost full moon at the solstice is quite common. So there is nothing about the appearance of a strawberry moon that is special. It is only that it is full on the same day as the solstice.

This means the moon is in “opposition” to the sun, that is, in the opposite direction from the earth. Since the sun is at the summer solstice (in the constellation Taurus, the bull), the moon must be at the winter solstice (in the constellation Sagittarius, the archer).

Well, almost at the winter solstice. You will notice in the second diagram that the sun is 73.5 degrees above the horizon, while the moon is +/- 26.5 degrees above the horizon. Why the hedging about the moon’s angle? Well, the moon doesn’t orbit the earth exactly in the plane of the ecliptic. If it did, there would be a lunar eclipse and a solar eclipse every month. The orbit is tilted at about 5 degrees, and I don’t happen to know how much it was off the plane of the ecliptic on June 20, 2016, hence the hedging

Diagram showing direction of sunset and moonrise
Diagram showing direction of sunset and moonrise

 

Diagram showing sun at noon and moon at midnight
Diagram showing sun at noon and moon at midnight

Postscript: the orbit of the moon lies in a plane that is tilted relative to the plane of the ecliptic by a little more than 5 degrees. So there is a line where the two plane intersect, and if you extend this line, it points to a specific point in the sky, somewhere in the zodiac. This point slowly moves  through the constellations of the zodiac, making a complete trip every 18.6 years.

The reason for this is that the plane of the moon’s orbit precesses, just like the plane of the earth’s orbit. The moon is much smaller than the earth, so it precesses every 18.6 years, while the earth’s orbital plane precesses every 25,000 years, known as the precession of the equinoxes. This is why the north celestial pole used to be near Vega, but is now near Polaris.

I read that this 18.6 year cycle was important to several prehistoric cultures. We know this by analyzing various astronomical constructions they made. Just like solstice means “sun standstill”, the corresponding event for the moon is called the “lunar standstill.”

We pay dearly for living indoors in cities, as we have few occasions for observing the night sky. Prehistoric peoples knew a lot of astronomy!

2 thoughts on ““Strawberry Moon””

  1. Your diagrams give a very good explanation of the situation. It is not that easy to take it all in. Wonder if they teach this stuff in school these days?..And isn’t it lucky that our spin axis is tilted!

    Strangely enough we were up at Jones farms in Shelton a few days ago picking strawberries…27 lbs. of the best ones we have seen in years… wonder if the berries know it’s their moon?

    1. Golly, if an engineer has trouble taking it in, no wonder my family does! You always know something best when you work it out for yourself: that’s why school works. I wish I knew how to be interactive online – out of my league.

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