UPDATE: 19 Sept 2015, 5:00 p.m. – Sadly, the weather is not cooperating with us this evening, so our IOMN session downtown in CANCELED. We’ll hope for better conditions during the lunar eclipse next week.
UPDATE: Meetup.com and Facebook Events have been added for both the IOMN (meetup | facebook) and eclipse IOMN (meetup | facebook) sessions.
Greetings fellow astrophiles!
We focus on the Moon this month with one natural event and one “nature-derived” (sounds better than “artificial”) event.
International Observe The Moon Night – Saturday, Sept. 19th, 7 – 9 p.m.
It’s the Moon, so doesn’t much matter where you set up to observe. A snapshot from last year’s CNYO IOMN session in Armory Square (near The MOST and Sound Garden).
International Observe the Moon Night (InOMN) is an annual event that is dedicated to encouraging people to ‘look up’ and take notice of our nearest neighbor, the Moon. From looking at the Moon with a naked eye to using the most sensitive telescope, every year on the same day, people from around the world hold events and activities that celebrate our Moon. On this site, you can find information about an InOMN event near you or register your own event. We encourage everyone to join us in the celebration!
Because the viewing was easier (and the crowd a little easier to find) from close to The MOST last year, we’re going to set up the scopes at (or close to) the very beginning of the south end of the Onondaga Creekwalk (map below, right below The Sound Garden). This spot provides ample parking and a fairly clear view of the Southwest/South/Southeast (certainly enough for lunar viewing) while not being quite as bright as other spots in the vicinity.
The 6 day old waxing crescent Moon is a nice compromise of brightness and detail for giving the Moon a good looking at (given the preference to have IOMN on a Saturday night, anyway). Not only will we have a terminator to give us shadows and perceived depth, but we’ll have pleasant views of the many large “seas” on the Moon’s surface – including Mare Tranquillitatis (with the Apollo 11 landing site just on its coast), Fecunditatis, Serenitatis, and Crisium – out in the open for inspection. For those wondering about the timing (besides the whole weekend thing), Full Moon is actually one of the most boring times to observe the Moon. With the Sun’s light beating straight down on the Moon’s surface, we have no shadows to bring out crater depth or mountain height. Most observers agree that the most interesting views are right along the terminator where light and dark meet, so having a nice piece of that to observe makes for a much more visually appealing session.
Total Lunar Eclipse – Sunday, Sept. 27th, 8:11 p.m. to 1:22 a.m. (28th)
NOTE: Bob Piekiel will be hosting a total lunar eclipse session at Baltimore Woods on the 27th. If you want to see the Moon in fine detail through telescopes, this will be an excellent place to be.
Those who’ve been keeping constant track may recognize the eclipse discussion below as a re-post from April, 2014 (Total Lunar Eclipse, Mars Just Past Opposition And A Very Early Observing Event At Baltimore Woods on April 15th), itself followed up by another lunar eclipse post from October, 2014 (CNYO Observing Log: Lunar Eclipse And Syracuse Academy Of Science, 8 October 2014).
And now onto the upcoming total eclipse – and my continued belief that lunar eclipses don’t get the respect they deserve. Yes, solar eclipses are much more exciting and it has been well-documented that people have previously responded very strongly (and not always pleasantly) to solar eclipses. The sudden darkening of the sky and noticeable temperature drop can cause all shades of responses (no pun intended) in people. That said, all we really get (besides a view of the solar corona) is an example of what happens when you put a black disc in front of the Sun. Lunar eclipses, on the other hand, tell us a bit about how the Earth itself interacts with the Sun by how this interaction alters our view of the Moon.
Both solar and lunar eclipses tell us something about the Sun/Earth/Moon relationship. Specifically, we learn that the Sun/Earth orbital plane (the oval made as the Earth goes around the Sun each year) and the Earth/Moon orbital plane (our local oval) are not the same – the Earth/Moon plane is tilted slightly off the Sun/Earth plane by 5.2 degrees (small, but just enough). That is, the Moon spends some time above and some times below the Sun/Earth orbital plane, while sitting right in the plane only two times each orbit (where the two planes intersect). How do we know this? Simple. If the Earth/Moon plane were exactly in the Sun/Earth plane, there would be a total solar eclipse and total lunar eclipse every month because there would be a time each month (New Moon) when the Sun, Moon, and Earth made a straight line (Sun-Moon-Earth = solar eclipse) and a time each month (Full Moon) when the Sun, Earth, and Moon made a straight line (Sun-Earth-Moon = lunar eclipse). As the two planes are slightly off, the New Moon is simply “off the radar” of most people because it can’t be seen during the daytime. The Full Moon, on the other hand, is brilliantly bright most of the time because it only infrequently enters the Earth’s shadow.
The image below shows this very nicely (and it’s always better to find and cite a good image than to have to roll your own). Give it a look for 30 seconds to make sure each of the four cases make sense to you.
The Sun/Earth and Earth/Moon orbital planes. Note the top and bottom orientations that are perfect for eclipses (and the left and right that are not). Image taken from www2.astro.psu.edu (from Chaisson & McMillan Publishing). Click for a larger view.
Total solar and lunar eclipses, then, occur on special, but periodic and predictable, occasions when the Moon finds itself exactly in the Sun/Earth plane. When it’s just ever-so-slightly off this plane AND still between the Sun and Earth (or still falls into the Earth’s shadow in the Sun-Earth-Moon arrangement), we get partial eclipses. Just that simple.
What to expect on April 15th (the government’s cashing in on its short wavelength tax!). Image from this article at io9.com.
Perhaps the most striking difference between a solar and lunar eclipse is that a solar eclipse obstructs the disc of the Sun, leaving only a view of its wispy exterior (corona), while a lunar eclipse alters the color of the Moon while still allowing us to see it in its entirety. Those watching the lunar eclipse will see the Moon go from its usual bright grey to orange, then a dark red before reversing the color order. The reason for this dark red coloring is the same reason why our sky is blue – the scattering of light in our atmosphere. Recalling our handy scattering relationship – that scattering (I) is proportional to 1 / wavelength4, we see that shorter wavelengths scatter more than longer wavelengths (because the wavelengths are in the bottom of the proportion, so larger numbers decrease the value of “I”). The image below was taken from one of the great non-wikipedia physics sites (well worth several afternoons to explore), hyperphysics.phy-astr.gsu.edu.
We see that shorter wavelength light gets “bounced around” more, while longer wavelength light passes for longer distances unimpeded by interactions with molecules and larger particles (like soot after big volcanic eruptions) in our atmosphere. Light going straight from the Sun hits our atmosphere and gets increasingly scattered as the wavelength gets shorter – blue scatters more than red, so we see the blue strongly when we look up during the day. With the blue light strongly scattered, those people on the edges of where the Sun’s light falls – those just starting or ending their days – see more red light because that wavelength wasn’t as strongly scattered – effectively those at sunrise and sunset get the filtered-out leftovers of the light that those at high noon see as blue. The “lit” side of the world experiences a range of different colors depending on where they are during the day, but all are being illuminated by waves of light from the Sun that left at the same exact time (plus or minus a nanosecond or two).
Because it’s a busy week and the author is feeling lazy, he refers you to the top image of the three-panel image below, showing how the scattering of sunlight in our atmosphere occurs sooner after entry (on average) for blue, a bit later (on average) for green, then a bit later (on average) for yellow, then out to red, some of which is and isn’t scattered (on average).
The scattering of light by Earth’s atmosphere (shorter wavelengths scatter sooner). The other two images are placed into context by your reading about extrasolar planetary atmosphere studies. See www.universetoday.com/…-in-blue-light/ for that info.
And so, we know that blue is scattered strongly and red is not. This red light then races to the edges of our illuminated globe and the red light not scattered directly down to Earth or scattered in the opposite direction (out into space right above you) races past Earth at various altered (scattered) angles. During the most complete part of the lunar eclipse, the red color you see is, in fact, the red light that is passing through the edges of our atmosphere at those places experiencing sunrise and sunset (the sunlight performing a “grazing blow” of our atmosphere). As you might guess, if Earth were to lose its atmosphere (but don’t give any of your industrious friends any ideas), our lunar eclipses would appear quite different. Instead of a dark red Moon, we’d simply see a black disc where no stars shone (like placing a quarter at arms length and obscuring anything behind it).
