A big full moon vs. the Perseids

The full moon rising at its perigee behind Mount Royal and St. Joseph’s Oratory, March 19, 2011. Near the horizon, the light of the moon is redder and softened as it passes through a thicker atmospheric layer.
Credit: Espace pour la vie (Marc Jobin)
Full moon rising at perigee © Espace pour la vie (Marc Jobin)
A big full moon vs. the Perseids

On August 10, 2014, at 2: 09 p.m. Eastern Time, the moon will be full. But this full moon will be something special: it will coincide, give or take a few minutes, with when our satellite arrives at the nearest point to the Earth in its orbit. A few hours later, when the sun sets in the west in Québec, the biggest full moon of the year will rise in the east.

In effect , the orbit of the Moon is not a perfect circle but rather an ellipse. During the month, the Earth-Moon distance varies between 356,000 kilometers at its perigee and 406,000 kilometers at its apogee. The full moon of August 10 will therefore appear 14 percent larger and up to 30 percent brighter than when it occurs at the furthest point of its orbit. But make no mistake: with the naked eye, you cannot see the difference between this and an “ordinary” full moon! In fact, the prevailing weather conditions – clouds and mist – will have a greater effect on the apparent brightness of the moon. As for its size, the famous “Moon illusion” (see box) will always give you the impression that it’s huge when it rises; how close it is makes no noticeable difference! Without a constant point of reference, we can therefore not rely on our eyes alone to properly assess the size of the Moon. But here’s a simple experiment that will allow you to see the change in size between the perigee and apogee: just take a picture with a digital camera with a good telephoto lens or coupled to a small telescope. With the same camera and the same zoom settings, repeat the exercise on the full moon of March 5, 2015, which will occur when the Moon is at its apogee – its farthest distance from Earth. You’ll then just have to compare the two images on the same scale.

High tides

The apparent size difference of the perigean full moon is too subtle to be perceived without a measuring instrument. On the other hand, its effects on tides are very concrete and directly observable – rain or shine! If you happen to be at the seashore during the period from August 10 to 14, be on the lookout: for a few days around the full moon, the ocean tides will be the highest for the whole of 2014. For example, the Minas Basin area (Nova Scotia) at the head of the Bay of Fundy is famous as the place where the tides are the highest in the world. During this period, the tidal range (difference between high tide and low tide) will be greater than 16 meters and approach the maximum possible for fair weather. If a storm adds itself to the equation, the tide could reach 20 meters!

The Perseids Suffer

Perigee or not, the light scattered by the full moon will cause collateral damage. The primary victim of this situation will be the Perseid meteor shower, probably the most anticipated astronomical phenomenon of the summer, whose maximum activity will take place in the early evening of August 12. When night falls, the Moon will have risen. Its brightness will overpower that of the smaller meteors, which are also the most abundant. Only the brightest will manage to pierce this veil of light pollution. Instead of the 60 meteors per hour that are seen under ideal conditions, we will see less than a dozen. Enjoy the show!

Did you know?

The illusion of the Moon makes it look immensely large when it is near the horizon at sunrise or sunset, when in reality it is the same size as when it is higher in the sky. One explanation is that the brain sees the sky not as a dome, but rather as a kind of ceiling parallel to the ground, a flat roof that is lost on the horizon. In everyday life, objects over our heads, such as clouds, are closer to us than those on the horizon. But the Moon being located well beyond the atmosphere, its image always strikes at the same angle in the eye, about half a degree. The brain interprets this image as a “small but close” Moon near its zenith, and a “huge but distant” one on the horizon. Imagine – the brain is so easily fooled!

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