image: NASA (  link  ).

image: NASA (link).

The moon is waxing towards full, and will reach the point of full moon on January 31. As you've probably already heard, this full moon will feature a total lunar eclipse, which will be visible as the moon is rising for viewers in eastern Europe, Asia, Australia and the western Pacific Ocean, and as the moon is setting for viewers in the eastern Pacific Ocean, and the western part of North America and Mexico (but the moon will pass through earth's shadow close to or after the moon sets for the eastern side of North America and for much of Central America, South America, Africa and western Europe).

The diagram above shows the times that the earth's shadow will envelope the moon, using the Pacific time zone in effect in western North America (including California). Viewers in the Eastern time zone of North America will have to add three hours to the times for the moon's entry into the penumbra and umbra -- which means that the moon will already have moved below the western horizon (moon set) by the time it is fully enveloped by the umbra for viewers in eastern North America, South America, the Atlantic, Africa and western Europe (see map below, also from NASA):

eclipse data jan 2018b.jpg

A total lunar eclipse can only occur when the moon passes through the point of full moon -- that is, when it is directly opposite from the sun: when the sun, the earth, and the moon are aligned such that the earth is between the sun and the moon. However, because the plane of the moon's orbit around the earth is tilted slightly from the plane of the earth's orbit around the sun, the moon will not go into earth's shadow every time that the moon reaches the point of full moon (when earth is between sun and moon). The moon will only pass into the shadow of the earth when the moon happens to reach the point of full moon at the same time that the moon on its tilted orbital plane is also passing through the plane of earth's orbit around the sun.

Further discussion of the moon's tilted orbital plane, and a few helpful videos to help you visualize what it means when the moon passes through the orbital plane of the earth around the sun, can be found at previous posts such as "The lunar nodes and the nine-world cosmology" and "An excellent website explaining the lunar nodes (with animation)." 

There is also some discussion of the January 31 lunar eclipse in this month's installment of "Skies over Grimerica," which was recorded on January 17th.

The leading edge of the moon will enter the penumbra of earth's shadow at 2:51 in the a.m. on early Wednesday morning, January 31, for observers in the Pacific time zone in western North America (this contact of the leading edge of the moon with the penumbra is also referred to as "P1"), and the leading edge of the moon will enter the umbra of earth's shadow at 3:48 in the a.m. just under one hour later (this contact of the leading edge of the moon with the umbra is also referred to as "U1"). The umbra is the full shadow of the earth, which would completely blot out the moon from our view, except for the fact that our planet has an atmosphere which bends the rays of the sun, allowing some rays to "leak through" to the face of the moon, giving it some illumination instead of plunging it into total blackness. These refracted rays are reddish in color, for the same reason that the sun and the moon appear redder when they are just rising or setting (because the rays of light are bending through our earth's atmosphere). 

There is a good discussion of this phenomenon, along with some nice diagrams, in this discussion of the January 31 eclipse over at Sky & Telescopehere.

As the moon continues on its orbit, the umbra will creep across more and more of the face of the moon, until the umbra reaches the trailing edge of the moon at 4:51 a.m. on Wednesday morning, January 31, for viewers in the Pacific time zone (the contact of the trailing edge of the moon with the umbra is also referred to as "U2"). At this point, the lunar eclipse is total. However, the centerline of the moon will pass through the very center of the umbra at 5:29 am for viewers in the Pacific time zone -- this is the point of "max eclipse" for this lunar eclipse.

The leading edge of the moon will begin to exit the umbra at 6:07 am Pacific time, which is also referred to as "U3." The moon will set below the western horizon for viewers in much of the west coast of North America before its trailing edge moves out of the umbra when the trailing edge of the moon leaves the umbra it is also known as "U4").

Viewers in other parts of the world which can see the lunar eclipse can use the Greenwich or Universal times shown in the upper-right corner of the second diagram above in order to calculate the local times for P1, U1, U2, U3, and U4.

The media is giving all sorts of sensational names to this lunar eclipse, in an attempt to make this month's event seem more unusual than other lunar eclipses -- but much of the hype involves the fact that this is the second full moon of the calendar month of January, which is simply an effect of the calendar and not of the celestial mechanics of the earth and the moon. Moreover, the use of the term "blue moon" to refer to the second full moon in a month appears to be relatively recent and -- according to this article in Sky & Telescope -- this use of the term "blue moon" may actually have resulted from a mistake by a writer in that same magazine back in 1946.

However, don't let the media's constant need to hype everything in their grasp spoil the experience of observing the eclipse for yourself, if it is at all possible for you to do so and if you live in a part of the globe where this eclipse will be visible during the time that the moon is moving through the umbra of earth's shadow. I believe that it is always beneficial to observe the motions of the heavenly cycles, and to marvel at the effects of the interplay between the moon and the earth and the sun. 

The moon is swinging along its path this very moment, heading for the point where it will intersect with the plane of the earth's speeding orbit around the sun, and in just over twenty-four hours it will begin to enter into the shadow cast by our entire planet. As always, that will be something to see.