The first total eclipse of the moon visible in North America since 2011 will occur April 14.
At 9:53 p.m. the full moon will enter the Earth’s shadow, beginning the eclipse. It will end nearly six hours later at 3:40 a.m. April 15, when the moon emerges in full from Earth’s shadow.
This is also the first of four total lunar eclipses in the next two years, coming at six-month intervals: April 14 and Oct. 8 in 2014, and April 4 and Sept. 27 in 2015. It will be about 20 years before we get four total eclipses in that kind of a time span again.
As the light from the sun passes Earth, it creates a shadow in space that has two parts: an outer, dimmer shadow called the penumbra, and a darker inner shadow called the umbra. In a total eclipse, the moon passes through the umbra.
A total lunar eclipse has five stages that are marked by the times the moon enters and leaves the penumbra and umbra. The first stage, called penumbra, begins when the leading edge of the moon enters the penumbra. You will not be able to observe this event; the change in the moon’s brightness is not detectable by the human eye. You will, however, be able to detect a pale shading on the leading (eastern) edge when the moon is about halfway across the penumbra.
The moon enters the darker umbra at 10:58 p.m. April 14, starting the second stage of a total eclipse: the partial eclipse stage. No direct sunlight passes through the umbra, so as the moon enters this part of the shadow a red-black line creeps, minute by minute, across the surface, leaving the moon less bright as it goes. This is one of the most visually impressive astronomical events in the night sky.
As the moon passes into the umbra, the rest of the sky falls into a deeper night, with more stars becoming visible on a darker background. An hour or so from the time the moon first entered the umbra, the last sliver of the bright moon disappears; the moon is now completely within the dark umbra. This is the beginning of the third stage, the total eclipse.
The total eclipse begins just after midnight, at 12:07 a.m. April 15. The moon will glow a deep orange or red during this phase. The reason for this color is the ring of sunrises and sunsets around Earth. Our atmosphere scatters and refracts sunlight that grazes the rim of Earth and causes the red light to enter the outer shadow. This means that the darkest part of the shadow is in the center of the umbra.
The shadow is large enough that the moon can pass above or below this dark center; and when it does, the refracted red light gives it the orange-red color. If the moon passes through the middle of the umbra, it will be darker, sometimes becoming almost black.
During the April eclipse the moon will pass south of the shadow’s center, with the northern edge just 1.7 arc-minutes south of the center. At the same time the southern edge of the moon will be 40 arc-minutes from the center. With careful observation you should be able to see a darker northern moon and a brighter southern moon at greatest eclipse, which occurs at 12:46 a.m., with changing surface brightness before and after greatest eclipse.
The total eclipse phase lasts for 1 hour 18 minutes. At 1:25 a.m. the eastern edge of the moon enters sunlight again. This is the beginning of the fourth stage, when the eclipse again becomes partial. The moon appears about the same as it did during the earlier partial phase, before the total eclipse occurred.
When the moon has completely escaped the umbra and only the shading of the penumbra remains, it has reached the fifth stage of the eclipse. At 2:33 a.m. the leading edge of the moon leaves the penumbra and the moon starts its move into full sunlight. By 3:40 a.m. the moon has left the Earth’s shadow, and the full moon is as bright as ever.
A total lunar eclipse is a great event, one that you only need your eyes to watch. No extra equipment necessary. Furthermore, the moon is not hard to find. And if the night of April 14-15 is cloudy, we only have to wait six months for the next opportunity for a total eclipse .
Marty Scott is the astronomy instructor at Walla Walla University, and also builds telescopes and works with computer simulations. He can be reached at firstname.lastname@example.org.