Penumbral Lunar Eclipse Feb 10
Eclipses are caused by shadows. The Sun sends light out in all directions and during a lunar eclipse, the Earth blocks the Sun’s light and the Earth’s shadow falls onto the full Moon. Things have to be lined up perfectly, so eclipses don’t happen every month, -the most we get is seven a year, about half of each type. In 2017, we get two solar and two lunar eclipses, a bit fewer than average.
Lunar eclipses would happen more often except that the Moon’s orbit around Earth is tilted and Earth’s shadow goes over or under the Moon most times. Secondly, there are two parts to the Earth’s shadow that can cover the Moon, an easily-seen dark central area, the umbra, and a surrounding penumbra, but it allows most of the sunlight through and is barely detectable. A penumbral eclipse is what we will get on Feb 10.
The shadow of Earth extends past the Earth and forms a dark cone stretching out to a point about 1,400,000 km away in space. That’s almost 4 times as far as the Moon is from us, so at the Moon’s distance, the shadow’s cross-section (a circle) is still big, almost 3 times as wide as the full Moon. For that reason, lunar eclipses last several hours and solar eclipses, with a much smaller Moon shadow, -only a few hundred km across- last only a few minutes.
The shadow of Earth on Feb 10 comes close but does not actually contact the umbra (it gets 99% of the way there). The second lunar eclipse this year, on August 7, is a bit better but still only partial and worse yet, for the whole eclipse, the Moon is below our horizon, -the best view will be in the Middle East.
The one bit of good news for Feb 10 is that it may be possible to see a darkening due to the slightly darker, inner part of the penumbral shadow. The outer penumbra is just too faint to see when it touches the face of the Moon at 5:34 pm. By 7:44 pm, however, a darkening on the upper left edge of the Moon (about 10:30 on the clock) may be discernible for perhaps 20 minutes either side of the prime time. The only way to tell for sure is to take a picture at 7:44 pm and compare it to another taken two hours later when the eclipse is over and the Moon is back to full brightness.
If it is clear Friday night, Feb 10, the Fox Observatory at the Outdoor Education Centre near Oliphant will be open to the public and members of the Bluewater Astronomical Society will be there to show you the Moon. You are welcome to take a souvenir photo through the telescope, -even a cell phone will work. If the weather cooperates, there will be time to observe the eclipse, take photos and have a hot chocolate to ward off the chill. Do join us.
Here is an image from the April 4, 2015 lunar eclipse taken before umbral contact to simulate the appearance of the Feb 10 event. Notice the darkening in the upper left of the Moon's disk, -this is about where to expect darkening for the Feb 10 eclipse. This image was taken 10 minutes before umbral contact and the inside edge of the penumbra should be darker than this for Feb 10. Skies will need to be perfectly clear!
Image by John H. Apr 4, 2015, Canon 60Da, 400 mm telephoto at f/5.6, 1/250 s, ISO 200
PS: The next good total lunar eclipse visible locally happens in two years on Jan 21, 2019 but only a year from now, on Jan 31, 2018, there is a good lunar eclipse visible from the west coast of North America if you are free to travel then. (I hear the skiing is good out there at that time, too). Mark your calendars!
Maps to the Fox Observatory are found on our HOME page.