Last night we experienced the first total lunar eclipse since Dec. 10, 2011. I'm writing this in advance of the lunar eclipse, and the forecast for watching it isn't looking good.
Don't worry, though this is the first lunar eclipse of a group of four total for this year into 2015. This eclipse was best viewed around 2 to 5:30 a.m. this morning for us on the East Coast, and started around 11 p.m. into the morning hours of the 15th for the Pacific Coast.
From the data I'm looking at currently the entire East Coast will be missing this event, while central and western America seem to have a better chance of clear skies. One of the best parts about observing a lunar eclipse is that you don't need any special equipment to view them since it's just a shadow covering the moon. That's unlike a solar eclipse where you need a special filter, or lenses in order to view.
So what is a lunar eclipse? Well, a lunar eclipse is when the moon passes directly into the Earth's shadow, cast by the sun. This only occurs when the Moon, Earth, and the Sun are in a straight line, or syzygy. The three bodies can only line up when the moon is full, hence why you may have noticed you've never seen or heard of an eclipse during any of the other phases of the moon such as a half moon.
Earlier I had mentioned that this is the first total lunar eclipse since 2011, but there are other types aside from the total eclipse. You may not even notice one type of eclipse known as the penumbral eclipse. Penumbra is Latin for almost, or nearly, and umbra is shadow, so when the moon is in a penumbral eclipse that means it is almost in the shadow/umbra. This type of eclipse is very faint and not very well noticed by the average person looking for an eclipse. I have a hard time noticing a difference between the regular full moon and the penumbral eclipse moon. This area of the Earth's shadow some of the lights source, the sun, is obscured by the Earth giving only a slight shadow or dimming of the moon. There are also total penumbral eclipses which means that the moon is in only the penumbra of the Earth. Total penumbral eclipses are quite rare, and even during this you may be able to spot a darker section of the moon which is closer to the darkest part of the shadow.
There can also be what is known as a partial lunar eclipse, which occurs when only part of the moon is within the umbra and the rest of the moon is in the penumbra. This is because the Sun, Earth, and Moon aren't in syzygy leaving the moon slightly higher, lower, to the right, or to the left of the Earth's shadow. The moon moves at about 2300 mph, and depending on it's place in orbit (distance from Earth in the orbit) the eclipse could last anywhere from an hour to four hours. If the moon is in apogee, the furthest point in orbit, then this will increase the length of an eclipse, and the opposite if the moon is in perigee, the closest point in orbit. When the moon is at apogee it is moving at a slower speed in its orbit than when it is closer in perigee.
An eclipse I have never witnessed is when you can see both the sun and the moon in opposite parts of the sky at the same time. Both sun and moon low on the horizon with a partial eclipse. This type of eclipse is known as a selenelion or selenehelion, or typically referred to as a horizontal eclipse. This is made possible due to our atmosphere refracting the light and making both the moon and the sun appear higher in the sky than they actually are.
During a total lunar eclipse like last night's, the moon doesn't completely disappear in the shadow, even though it is in the umbra. Light refraction through Earth's atmosphere from the Sun actually illuminates the face of the moon giving it a dark red color. Because the sunlight is passing through a large area of our atmosphere it is going through particles and pollution giving it that red color you see. This is the same reason for the orange or red you see lighting up the moon during a moon rise or moon set. The red color of the moon during an eclipse can vary from different shades of red. The more dust and clouds in the atmosphere the darker the red color.