Super Eclipse: Giant Harvest Moon Rises Blood Red, Produces Giant Tides, Disappears

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Even in total eclipse, the moon will appear red. Photo courtesy of NOAA.

Even in total eclipse, the moon will appear red. Photo courtesy of NOAA.

You might be tempted to sing to the rising harvest moon Sunday, September 27, but no matter how loudly you exhort it to ‘shine on,’ it will disappear — at least partially for three hours, and completely for over an hour.

Some natural occurrences involve gas, liquid, or solid. This total eclipse of the moon involves all three. Luckily all the related phenomena are governed by the same physical laws.

What Is A Supermoon?

The moon circles the earth in an approximately circular orbit: Approximately, but not quite.

The moon’s average distance from the earth is 384,000 kilometers (238,000 miles), but at perigee (closest approach) it is only 357,000 kilometers (222,000 miles) away.

At perigee, the moon appears 14% larger (diameter) than at its average distance.

Since a 14% larger diameter means a 30% larger area (Area=πr^2; r is the radius, which is half the diameter), and since the sunlight reflected by the moon to the earth depends on its area, the Supermoon appears 30% brighter than average.

What Causes A Lunar Eclipse?

Eclipses occur when the sun and moon are aligned with the earth in a straight line.

  • When the moon is between the sun and the earth, there is a solar eclipse.
  • When the earth is between the sun and the moon, there is a lunar eclipse.

It is an astronomical coincidence that the apparent size of the moon and sun as seen from the earth are almost the same — the moon is closer but the sun is bigger. When a solar eclipse occurs during a Supermoon, it is total; but if the eclipse occurs when the moon is at apogee (farthest from the earth), the eclipse is annular — a ring of the sun is visible outside the moon.

Since the shadow of the earth is several times wider than the moon, the Supermoon does not affect an eclipse of the moon very much.

If the moon revolved around the earth in the same plane as the earth revolves around the sun, there would be an eclipse of the sun and an eclipse of the moon every month. But the orbit of the moon is tipped about five degrees from the plane of the ecliptic (the plane of the earth’s orbit), so the three bodies don’t line up that often. Since the moon is only at perigee one day per month, an eclipse during a Supermoon is quite rare: The last one was in 1982; The next one will be in 2033.

Eclipses And The Ancients: The Shape Of The Earth

The Greeks knew something about eclipses 2,000 years ago. They suspected that eclipses were the result of the earth passing between the sun and the moon and that they were seeing the shadow of the earth on the moon. That shadow was curved, so they suspected the earth was round. It took 1,600 years for the idea to gain general acceptance.

Supermoon Rises Big And Red

The moon or sun near the horizon often appears reddish. Thank the British Lord Rayleigh (John William Strutt), who found that the scattering of light by air molecules is inversely proportional to the fourth power of the wavelength. In simple terms, shorter wavelengths, those near the blue end of the spectrum, are scattered most, while longer wavelengths, red and orange, are scattered least.

The sky is blue because the blue wavelengths are scattered the most.

The sun and moon near the horizon appear red because the light travels through a long expanse of atmosphere and the red is scattered least. The effect is especially pronounced for the moon at the horizon near the time of full moon because both the sun’s light going to the moon and the light reflected by the moon to the earth pass through a maximum amount of atmosphere.

Even in total eclipse, the moon will have a red glow from the light that reaches it after being bent slightly by the earth’s atmosphere.

Supermoon, Supertide

At the times of full and new moon, the sun and moon combine their gravitational pulls on the earth. The solid earth responds only minimally, but the ocean tides are pulled to their maxima for the year (spring tides, also now called king tides). Flooding is expected in low-lying areas. Along the east coast of the United States, where easterly winds are already causing high waves and tides, the effect will be exacerbated.

Where To View The Supermoon Eclipse

Most of the United States will have good eclipse-viewing conditions Sunday night. Forecast courtesy of NOAA.

Most of the United States will have good eclipse-viewing conditions Sunday night. Forecast courtesy of NOAA.

Some part of totality will be visible (weather permitting) in all of North America, South America, Africa, and Europe. The eclipse will not be visible in most of Asia or in Australia. The place that will have the maximum totality is in the middle of the Atlantic Ocean.

Much of the United States will have good viewing conditions, but the southeast will probably be cloudy and/or rainy.

Western Europe should have good viewing.

Wherever you are, check the latest forecast because local conditions can produce weather that is different from that in the general region.

When To View The Supermoon Eclipse

In London the eclipse will start about one a.m. Monday morning, and in the rest of Europe it will be at least that late. But on the east coast of the United States, this eclipse could not be more convenient. A sliver of the moon will disappear at around 9 p.m. – totality begins around 10, and the whole eclipse is over about 12:30, just in time for the harvest moon to shine on ‘for me and my gal.’

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