Hello everyone, I’m Darrell Heath with the UALR College of Arts, Letters, and Sciences; welcome to The Night Sky.
September of 2015 will go down as one of the best months of the year for Moon lovers of all kinds. Not only do we have International Observe the Moon Night going on, but we also have a total lunar eclipse of the full Harvest Supermoon.
International Observe the Moon Night is an educational outreach event sponsored by various NASA and astronomical organizations and occurs on September 19th. It’s an educational outreach event sponsored by various NASA and astronomical organizations in order to get people outside and looking up at our nearest celestial neighbor. On this night astronomy clubs, planetariums, museums, and science centers all around the world will have telescopes set up for anyone curious enough to come out and see the Moon close up.
The moon’s phase for this evening is a 5-day-old waxing crescent. You may be wondering why we aren’t looking during a full Moon phase. Well, a full Moon is actually the worst time to view the Moon with a telescope. During a full Moon the entire surface is illuminated and the Sun’s light makes everything look flat and one-dimensional. The best time to see the Moon through a telescope is when it is partially lit and the best place to look is along the terminator. No, not the Arnold Schwarzenegger variety but the line that divides the Moon into its day and night sides. If you were standing on the Moon along the terminator you would see the Sun rising or setting off to one side and its light would make the craters and mountains stand out in sharp relief. Viewing from the Earth you would also benefit from this effect and from night to night as the terminator moves you get to see more and more varied terrain features. On International Observe the Moon Night we will be able to see a number of the Moon’s maria, which are large, flat plains of solidified basaltic lava and they can be viewed in binoculars or even with the unaided eye.
There are other interesting lunar landforms that are favorable for viewing on the evening of the 19th as well. The Moon’s Caucasus Mountains, a mountain range some 520 km long and 6 km high will be viewable, as will the Valentine Dome, a low-profile volcanic dome just east of the south tip of the Caucasus’ and visible only when the terminator is near, like tonight. Seeing the Moon’s ancient and battered landscape through a telescope is always a thrilling sight. If you’ve never had the pleasure then you owe it to yourself to come out and take in the view.
The Central Arkansas Astronomical Society will be partnering with the Innovation Hub in North Little Rock on September 19th to celebrate the Moon. Telescope views, some of UALR’s finest meteorite specimens, science demonstrations, music, food, and more will be on hand at the Innovation Hub from 7-10pm. For more information about the event, visit the Night Sky website.
Moving on towards the end of the month we have a total eclipse of the full Harvest Supermoon on the night of September 27th. Maybe we should take that apart and look at each component in a bit more detail.
A full Moon is when the Moon appears fully illuminated from our perspective here on Earth. As the Moon and Earth orbit around one another we will only see a full Moon when the Moon is on the opposite side of the Earth from the Sun.
Harvest Moons are full Moons that occur either two weeks before or two weeks after the Autumnal Equinox, the first day of Fall in the northern hemisphere. Now, the interesting thing about Harvest Moons is that they rise about 30 to 35 minutes later than they did the evening before which, traditionally, has served to provide farmers with extra light to work by in order to bring in their crops.
Normally there’s a difference of about 50 minutes in between rise times for the Moon each night, so what makes the Harvest Moon so different?
Well, it’s at this time of year that we see a change between the angle of the ecliptic with that of our horizon. The ecliptic is the narrow path we see across the sky through which the Sun, Moon, and planets all appear to move over the course of the year. What we call the ecliptic is in fact a reflection of our Earth’s orbital motion around the Sun. Think of it as viewing our solar system’s plane edge on. When you combine the fact that we are moving around the Sun with the fact that our axis of rotation is tilted by 25 and-a-half degrees you see something very interesting: the ecliptic is never in the exact same place throughout the entire year and changes with the seasons. Around the time of the Autumnal Equinox each September we see that the ecliptic has a very narrow angle relative to the horizon. Just compare September’s ecliptic with that of March’s. In March the angle is very steep indeed, with a decrease in that angle we see a corresponding decrease in the times between successive moonrises and we get the phenomena we call the Harvest Moon.
The ecliptic gets its name from the fact that eclipses only occur along this path in the sky. Now, the Moon orbits around the Earth once a month so, all things being equal, you might expect to see an eclipse once a month as well. BUT, not all things are equal. The Moon’s orbital path around the Earth is tilted by about 5 degrees with respect to the plane of the ecliptic. This means that eclipses are infrequent and can only happen when the alignments between the Earth, Moon, and the Sun are very precise. On the night of September 27th those conditions will be met and we will see a total lunar eclipse when Earth’s shadow completely covers the Moon’s disc. Lunar eclipses can only occur during a full Moon and this particular one happens to be on the night of a Harvest Moon as well.
Provided the weather cooperates you can look at the start of the eclipse around 8:07PM with eclipse maximum occurring at 9:47PM. This early evening event means that it will be a great time to be outside with family and friends to observe this amazing celestial spectacle. You can see the event from anywhere that you have a clear view to the southeastern part of the sky. Check out the Night Sky website for more info regarding viewing locations.
Last but not least, this particular eclipse of the full Harvest Moon will also be what is known as a “supermoon”. Who says it’s a supermoon? Astrologers and the news media do and, so, astronomers (people who are interested in the scientific aspects of our universe) simply ignore the hype. According to astrologers, a supermoon is a full Moon that coincides with the Moon’s perigee: its closest point to Earth for the entire month. The Moon doesn’t orbit the Earth in a perfect circle; if it did then its position would always stay the same. Instead, the Moon’s orbital shape around the Earth is that of an ellipse, a somewhat flattened circle and its distance from us changes from day to day. You might think that a full moon that is closest to the Earth will be noticeably bigger than any other full moon; sadly this just isn’t the case. On this night the Moon is 221,754 miles away, closer than the average 239,000 miles distance, and, technically should be a bit bigger than most other full Moons but it’s just not significant enough for our eyes to even register the difference. Oh, and don’t worry, a “supermoon” is not going to cause earthquakes or knock your chakra out of alignment.
So, even though you may not be able to discern any difference in the apparent size of the Moon you can still experience the optical illusion known as “the Moon Illusion” on any night of or around a full Moon. For whatever reason our eyes and brain fool us into thinking that a Moon seen near the horizon is bigger than at any other time. But it is in fact just a trick of the mind, a giant-looking Moon seen near the horizon is no bigger than any other time you’ve seen the Moon. You can prove this to yourself by holding an average-sized aspirin between your thumb and forefinger out at arms length. Let it cover the rising Moon and then check back an hour or two later to repeat the experiment again when the Moon is higher up in the sky. You will see that both Moons are in fact the same in angular size and that nothing has changed.
No matter when you observe the Moon, if you do it carefully and really take the time to notice details about it you can’t help but look up in both awe and wonder.
