Hey! Friday morning is Groundhog Day! A cloudy day supposedly forecasts an early spring. But why does it fall on February 2 each year? That date is one of the four so-called cross-quarter days, which are the midpoints between the solstices and equinoxes. February 2 is midway between the winter solstice and the vernal equinox, so we expect the weather to start being more spring-like and less winter-like. On February 2, Roman Catholics celebrate Candlemas. In contemporary paganism, the day is called Imbolc, a traditional time for initiations. Astronomers don’t care too much if spring comes early — as long as the skies are clear.
A blue blood supermoon eclipse
On January 31, 2018, much of the world will receive a post-holidays gift — the total lunar eclipse of a full supermoon that is also a “Blue Moon”! As you can see from my heading, there’s going to be quite a bit of hype around this event. Let’s break it all down.
Because Earth is a solid sphere with sunlight shining on it, it casts a circular shadow into space opposite the sun. Of course, the black shadow on black space is invisible. But if an object, such as the International Space Station or the moon, travels through that shadow, no rays of sunlight can reach it, so it darkens. If you watch for passing satellites on a clear dark night, and note where in the sky they disappear (leave sunlight) or suddenly appear (enter sunlight), you can trace out the Earth’s shadow. It always there!
But some of the sunlight streaming past Earth passes through our atmosphere, and air refracts (bends) rays of sunlight. When we watch a sunset, we actually see an image of the sun for a short time after it has already dropped below the horizon! That bending allows sunlight to bend around the Earth’s horizon, turning an eclipsed moon reddish with sunset coloured light — hence the nickname “blood moon” for eclipsed moons. If you could view a total lunar eclipse from the surface of the moon, the round black Earth would have a 360° halo of reddish light!
The shadowed region where the sun is completely blocked by the Earth is called the umbra (that’s where the word “umbrella” — a sun shade — comes from). Surrounding this is a much larger, but weaker circular shadow called the penumbra. Some of the sun is still visible for objects passing through the penumbra. Lunar eclipses occur when the steady eastward motion of the moon in its orbit around Earth carries it though the penumbra and/or umbra.
The moon is half a degree wide in the sky, while the umbra and penumbra are about 1.5° and 2.5° across respectively. So, depending on the geometry of any given eclipse, the moon takes about 5 hours to cross the penumbra, spending about two of those hours inside the central umbra. During partial lunar eclipses, the moon never fully enters the umbra.
The moon is only full when it is opposite the sun in the sky, and the Earth’s shadow is always opposite the sun, too — so lunar eclipses can only occur on full moons. Even though the size of Earth’s shadow is large compared to the moon, the moon usually misses it. That’s because the umbra always lies near the ecliptic. The moon’s orbit is tilted about 5 degrees from the ecliptic. Lunar eclipses can occur only if the moon is full while it is near the point in space where the moon’s orbit crosses the ecliptic.
The January 31 total lunar eclipse occurs only 1.2 days after perigee (the moon’s closest approach to Earth), so the moon’s diameter will appear about 7% larger than average, making it super. This full moon will also be a Blue Moon — unofficially defined as the second full moon to occur within any calendar month. (February will not get a full moon this year.) Eclipsed supermoons aren’t all that rare, but the total eclipse of a Blue Moon hasn’t occurred since March 31, 1866. That’s 152 years! Don’t let the nickname mislead you, though. Blue moons don’t look blue at all.
The entire eclipse will be visible from northwestern North America, across the Pacific Ocean, and as far as eastern Siberia and Asia. Most of North America will see a portion of the eclipse before the moon sets and morning twilight arrives — while Eastern Europe and Central Asia will see the eclipse already in progress when the moon rises. During totality, the moon’s northern limb will pass just south of the center of the umbra, darkening the moon’s northern half more than its southern half.
The moon’s edge will start to darken at 6:48 am EST (Eastern Standard Time) or 11:48 UT (Universal Time). Maximum eclipse occurs at 8:30 am EST (13:30 UT), and the eclipse ends at 10:11 am EST (15:11 UT). For observers in the GTA, the moon will set at 7:34 am, before maximum eclipse. At that time, it will be about halfway into the umbra. So, now you know why this eclipse is blue, super, and blood red. Hopefully the skies will be clear wherever you are!
The Moon and Planets
The moon starts the week off as a waxing gibbous orb shining in the evening sky inside the stars of the Winter Football asterism. It’s at the feet of Gemini (the Twins) tonight (Sunday), and closer to the star Pollux on Monday, before exiting the football for Cancer (the Crab) on Tuesday night, when it will look full. It will formally reach full moon during the eclipse on Wednesday morning, and then appear close to full again when it rises near Leo (the Lion) on Wednesday night.
The dwarf planet (formerly asteroid) Ceres (“seer-ease”) reaches opposition on Wednesday, January 31. This is when it is at its brightest (visual magnitude 6.85) and closest to Earth for the year. It will be observable all night using binoculars or telescopes. At opposition, Ceres will be situated near the northern boundary of Cancer, about a fist’s width above the naked eye star Algenubi, which marks Leo’s nose.
For the early risers, Jupiter is the very bright object shining in the east after it rises about 2 am local time. It’s easily visible until dawn, when it’s well above the southern horizon. Reddish Mars (which rises about 3 am local time) is sitting about a fist’s width to Jupiter’s lower left, and increases that distance by a little bit every morning. It’s much dimmer than Jupiter now, but Mars will steadily brighten over the next six months.
Yellowish Saturn is about 4.5 fist width’s to the lower left of Jupiter. It rises before 5:30 am local time, but the best time to try for it is around 6:30 am. It is the only prominent object in that part of the sky — making it easy to identify. Saturn will continue to rise earlier and climb higher every morning during the next few months.
Keep looking up to enjoy the sky! I love getting questions so, if you have any, send me a note.
Astronomy Skylights for this week (from January 28th, 2018) by Chris Vaughan.