Guest guest Posted February 28, 2007 Report Share Posted February 28, 2007 Total Lunar Eclipse March 3, 2007 Total Lunar Eclipse There will be two total lunar eclipses visible from Ottawa this year. The more convenient one is coming up soon -- early evening on Saturday, March 3, 2007. Unfortunately, the timing is not ideal: the event will already be underway when the moon rises, so you'll have to be waiting for it or it may pass unnoticed. Here are the official timings as published by NASA (converted to Eastern Standard Time). The terminology is explained below. Penumbral Eclipse Begins: 3:18 PM Partial Eclipse Begins: 4:30 PM Total Eclipse Begins: 5:44 PM Greatest Eclipse: 6:21 PM Total Eclipse Ends: 6:57 PM Partial Eclipse Ends: 8:11 PM Penumbral Eclipse Ends: 9:23 PM 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. Unfortunately for Ottawa eclipse fans, the moon rises at 5:52 PM on this day, so the total eclipse will already be underway, and we will miss the exciting "first bite" phase. A totally-eclipsed moon is not invisible, but it will rise darkened to a deep musky orange colour. The darkened full moon will rise in the sky and at about 7:00 PM we will see the bright crescent reappear (in the lower left quadrant) and grow as the moon emerges from eclipse. It will be full again by 8:11 PM that evening, and fully bright by 9:23 PM. Next Eclipse The next, and only other, eclipse this year is also inconveniently timed. It starts at 3:50 AM on the morning of August 28. Observers who stay up late will be able to enjoy the "first bite" as the shadow begins to cut across the moon's full disk. However, the moon will sink below the Western horizon just as totality hits, so we won't see the total phase or the re-emergence of the bright crescent. 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. 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.) Article & Diagrams by Richard McDonald. Purity, Powers, Parabrahmam... Click to join Rajarajeshwari_Kalpataru It's here! Your new message!Get new email alerts with the free Toolbar. Quote Link to comment Share on other sites More sharing options...
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