Total Lunar Eclipse

(Published January, 2008)

There will be only one total lunar eclipse visible from Ottawa this year and, weather permitting, it will be an excellent one for viewing. (Although totality will be considerably shorter than during last year’s eclipse, the moon will be well placed in the sky and we will see the entire event.) It is on the evening of February 20, 2008.

Here are the official timings as published by NASA (converted to Eastern Standard Time). Note this eclipse happened in 2008 — these times do not apply to any eclipse that may be approaching now. They are left in this article to give an idea how long the phases can take. The terminology is explained below.

Penumbral Eclipse Begins: 7:35 PM
Partial Eclipse Begins: 8:43 PM
Total Eclipse Begins: 10:01 PM
Greatest Eclipse: 10:26 PM
Total Eclipse Ends: 10:51 PM
Partial Eclipse Ends: 12:09 AM
Penumbral Eclipse Ends: 1:27 AM

What we’ll see

The effects labeled Penumbral are not visible to the casual observer (see explanation below), so we are interested in the Partial and Total times.

At about 8:43 PM we will first see a “bite” being taken out of one side of the moon. Over the next hour this will grow in size until the moon is totally eclipsed at 10:01 PM. While totally eclipsed, the moon is not invisible, but will be a dark dusky red-brown. A sliver of bright crescent will re-appear about 10:51 PM, gradually growing until the fully bright full moon is restored a few minutes past midnight.

What’s Happening

A total lunar eclipse is the result of the Earth’s shadow crossing the moon’s disk. If you think about this, it’s clear that, for the Sun to be throwing the Earth’s shadow directly at the moon, the Sun and Moon must be on exactly opposite sides of the Earth. That’s also the definition of Full Moon, which is why total lunar eclipses can only occur when the moon is full. (Why isn’t there an eclipse every time the moon is full? That’s discussed below.)

Umbra & Penumbra

To understand the term penumbral as used in the timetable above, we need to think about the fact that the Sun is very large and is close enough to the Earth that it doesn’t act like a point source. From a given point of view behind the Earth, light from one side of the Sun may be blocked by the Earth while light from the other side of the Sun is still visible.

The area of complete shadow behind the Earth is called the Umbra. Anyone in the Umbra would not be able to see any part of the Sun.

In the Penumbra area, part of the Sun will be obscured by the Earth, but part will be visible. For example, someone near the word “Penumbra” in the diagram above can see the upper part of the Sun but not the lower part.

We say the moon is in total eclipse when it is entirely in the Umbra — in complete shadow. When the moon is in the Penumbra it is, technically, receiving less sunlight, but the Sun is so bright that the difference is not noticeable to a casual observer.

Why isn’t it Invisible? Why is it Red?

When the moon is in total eclipse, it is in the Umbra, which means it is in complete shadow. So why can we still see it? A moon in total eclipse doesn’t disappear, it just turns a very dark smoky red colour.

The moon appears red for the same reason that sunsets on the Earth appear red. The earth’s atmosphere contains particles (dust and moisture) that have the effect of scattering the light that passes through it. Red light is scattered the most (the amount of scattering is a function of the wavelength of the light being scattered as related to the size of the interfering particles).

As sunlight streams by the Earth, the thin layer of atmosphere scatters some of the red light, throwing it into the Umbra. So objects that should be in full shadow are, instead, received a small amount of red-coloured illumination from scattered sunlight.

When do Eclipses Happen?

As mentioned above, a total lunar eclipse requires a full moon, but a full moon does not guarantee a total lunar eclipse.

That’s because the plane of the moon’s orbit around the Earth is not exactly in line with the plane of the Earth’s orbit around the Sun. If these were precisely aligned, then every time there was a full moon there would be a total lunar eclipse.

Instead, the plane of the moon’s orbit around the Earth is tilted about 5 degrees from the plane of the Earth’s orbit around the sun.

The result is that the moon’s orbit frequently places it outside the Earth’s Umbra. Only when the tilted orbit happens to bring the moon in line with the Earth-Sun line does a total eclipse occur.

Viewed from somewhere on Earth, there are always at least 2 total lunar eclipses in a year, and there can be as many as 3. On average, there are 2.44 total lunar eclipses per year. (There is, of course, no guarantee that these eclipses are timed for the convenience of Ottawa observers. This year, only one is during our night time.)

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