Astro Bob: Total lunar eclipse coming this Sunday night
You won't want to miss Sunday night's total lunar eclipse. If you do, the next one won't be till May 26, 2021. The eclipse takes place between about 9:10 p.m. (Central Time), when we'll see the first hint of the Earth's outer shadow darken the mo...
You won't want to miss Sunday night's total lunar eclipse. If you do, the next one won't be till May 26, 2021. The eclipse takes place between about 9:10 p.m. (Central Time), when we'll see the first hint of the Earth's outer shadow darken the moon's edge, until 1:15 a.m. the next morning, when the moon breaks its alignment with the sun and Earth and eases back into sunlight.
A lunar eclipse can only occur during a full moon because full moons are always directly opposite the sun. From outer space, we'd see the sun, Earth and moon in that order. Earth casts a two-part shadow-a dark core called the umbra surrounded by the lighter penumbra. The penumbra isn't fully dark because sunlight filters into it, diluting the darkness. From the viewpoint of someone standing on the moon looking back, the Earth has only covered part of the sun. The moonscape is twilight-like but not yet dark.
During a lunar eclipse, the moon passes first through the penumbra and then into the much darker umbra. The umbra is only about 1.5 degrees wide or the width of your thumb held at arm's length toward the sky. For the moon to pass through that small target, the lineup has to be nearly exact. The "black bullseye" is always there, but because the moon's orbit is tipped 5.1 degrees to Earth's orbit, most full moons miss it, passing a couple degrees above or below the shadow.
By the way, Sunday's full moon will be a little larger than normal because it happens to be undergoing eclipse just one day before its closest approach to the Earth called perigee. That makes a supermoon, the first of three in 2019. Will it look bigger to your eyes? Probably not unless you've really been paying close attention to the moon's size during recent full moons. The average apparent diameter of the moon is 31 arc minutes or just a hair more than a half-degree. That's half the width of your little finger held at arm's length. Sunday's supermoon will be 2.2 arcminutes or 7 percent larger. Let us know if you can tell.
Unlike a solar eclipse where you have to take precautions to shield your eyes against the sun, you can watch every part of a lunar eclipse. You don't even need equipment to enjoy the view, but bringing binoculars along doesn't hurt. They enhance the eclipse colors and show stars shining right next to the full moon during totality - a beautiful sight only seen during an eclipse. Telescope users can track the shadow more closely and watch it cover the larger craters and lunar seas like a spilled tea slowly soaking into paper.
You'll first notice the penumbral shading around 9:10 p.m. (CST) or even a little earlier. It's not a "hard bite" like the umbra makes but more of a dusky shading along the left-hand (celestial east) side of the moon. 25 minutes later, the moon eases into the umbra for that first nibble of shadow. As it progresses deeper into the gloom, look for the first signs of color, usually orange or red. Two things about the shadow - it's curved and it's fuzzy. I bet you can guess why it's curved. If you said because the Earth is spherical and spherical objects cast curved shadows, give yourself 10 bonus points.
Earth's shadow has a fuzzy edge for the same reason a tree casts a fuzzy shadow. Because the Sun is an extended disk rather than a point of light, light from one side of the disk spills into areas that the other side of the Sun can't reach and vice versa. This "spillage" softens and diffuses the shadow's edge. Only point sources like Venus can create sharp-edged shadows.
Once the moon is fully engulfed in shadow and totality has begun, the lunar disk is awash in yellows, oranges and reds. This sight of the moon painted in vivid colors is often the highlight of the eclipse. It looks almost alien up there among the stars. If you're lucky enough to live where light pollution is low, you'll see all the stars and Milky Way return for an hour when the moon is inside the umbra. A totally amazing transformation!
If the Earth had no atmosphere the moon would be utterly black once fully within Earth's shadow. But our atmosphere bends the light of the sun - located in front of the Earth, remember? - and focuses it inside the shadow. Because the light grazes the edge of the planet exactly the same way it does at sunrise and sunset, those are the colors that light up the moon! To gaze at the eclipsed moon is to see the light of every sunrise and sunset around Earth's 360° perimeter. That's what I call poetry.
If the moon is relatively bright in Earth's shadow, it's a sign our planet's atmosphere is relatively dust-free. But if large volcanoes have been recently active, filling the atmosphere with ash and sulfur dioxide, the moon can appear unusually dark. Because the moon in this eclipse travels north of umbra's center, its top half will probably appear brighter than the bottom.
As the moon continues through the shadow traveling at an average speed of 2,288 mph (3,682 km/hour), it soon gets to the other side of the umbra and slowly emerges into the "partly sunny" penumbra again, signaling the end of totality. Another hour or so passes before it leaves the umbra.
The last lunar eclipse happened just shy of a year ago during the early morning hours of Jan. 31, 2018. I'd argue that this one will be a little easier to watch because it occurs during convenient evening viewing hours. Some of you will catch the entire eclipse, but if you're short on time and just want the juicy parts, be out from 10:15 p.m. to 10:45 p.m. (CST) to watch the end of the partial phases and beginning of the totality. For the Eastern Time Zone, that would be 11:15-45 p.m.; Mountain 9:15-45 p.m. and Pacific 8:15-45 p.m.
Lots of us will be taking photos. That's why I recommend Fred Espenak's Mr. Eclipse site for everything you need to know. If you are using mobile phone, be aware that the moon will be very small in a black sky in your photos. I recommend waiting until shortly before, during and after totality to take pictures. At those times, the ambient city light will be a better match for the moon, which will allow you to frame the moon with a night scene. Be sure you bring something to steady your phone like a bean bag, otherwise it will shake during the exposure, blurring everything.
Good luck and clear skies!