Each month I will describe sights of interest in the night skies of South Carolina. These sights will be broken down into four sections; what you can see with the naked eye, with binoculars, with a small refracting telescope, and with a Dobsonian reflector. The best time to view the night sky is at and around the times when the Moon is not visible, what is known as a New Moon; which will occur this month on October 30th. For October, your best viewing nights will be from October 1st to the 5th and the 20th to the 31st. The Star chart below is set for Florence, SC on October 15th at 9 pm. Note: A table of contents for earlier columns is located in the February 2016 column.
The Harvest Moon:
On many occasions, I will write a column based on questions I am asked by students or friends during the past month. This month, on four occasions I was asked about the Harvest Moon. I then checked and found that I have not discussed this subject since 2010. So for my new readers, I will revisit this interesting subject.
The short and simple answer is that a Harvest Moon is the full Moon closest to the Autumnal equinox; therefore occurring near the time that farmers harvest their fields. The rising full moon just after sunset would then allow them to extend the time of their harvesting by using the light of the full Moon. So this year, the true Harvest Moon occurred on September 16th.
However, there is a longer answer to this question. What few people know is that one to two days after the full Moon, the farmers can still use the Moon to extend their harvesting. This situation only occurs in the autumn, thereby extending the concept of a Harvest Moon. The reason is as follows. The Sun and all the planets are located on about the same plane. This is the reason that you always find the planets by looking in the region of the path of the Sun.
However, the plane of our Moon’s orbit is tilted versus the plane of the solar system. You know this to be true, because if the Moon’s orbit was on the same plane as the solar system, then each month we would have one solar eclipse, and one lunar eclipse. Instead, each month the Moon passes below or above the Sun, and on chance occasions, it eclipses the Sun. Now remember our Earth is tipped 23 ½ degrees, and as we circle the Sun, this affects our Earth/Sun perspective; known as the change of seasons.
Now, back to the Harvest Moon: Combining the above parameters, at any given month of the year, the angle of our Moon’s orbit versus the visible horizon will vary. This change results in an interesting effect on Moon rises. The full Moon in spring near the Vernal Equinox follows a path that results in a large angle of the Moon’s orbit to the horizon, see below.
This diagram can be somewhat confusing, however, direct your attention to the lower center left of the image. Note how far below the horizon the Moon is located each night after the full Moon. The farther the Moon is below the horizon, the longer it will take to rise on the next night.
Now, look below at the diagram for the full Moon in the fall near the Autumnal Equinox.
Again direct your attention to the lower left side of the image, and note the distance below the horizon the Moon is located each night after the full Moon. The Moon is relatively close to the horizon on the nights following the full Moon. Therefore, on the nights following the Harvest Moon, the Moon will rise only a short time later than the previous night.
So, during harvest, the night after the Harvest Moon, the farmer can continue harvesting in the twilight for about 20-25 minutes after Sunset, and again he will enjoy the Moonrise so he can continue his work.
After the springtime full Moon, on the next night the Sun would set, and darkness would come, and it would take a minimum of 60 minutes after sunset until the Moon rises.
We have had an extremely hot and humid summer, and in general, this is not favorable for good telescopic viewing. Finally, this month we should begin to have cooler less humid skies and much better telescopic viewing, with conditions continuing to improve throughout November and into December. Saturn will still be a good target low in the west. It is noteworthy that Saturn’s ring tilt will still be favorable for the next four to five years. During this time, Saturn viewing will extend onward through the clearer sky months of November and December.
You should also turn your scopes to the planet Venus after sunset in the west. Although Venus is covered in clouds, check out the subtle phase changes throughout the month. Near the end of the month you will see the Pleiades rising in the east, followed by Taurus and finally the beautiful constellation Orion will peak over the eastern horizon near the end of the month. For thousands of years people knew that this was the sign the winter is on the way.
Naked Eye Sights: Throughout the month you can watch brilliant Venus rise higher each evening in the west after sunset. Near the end of the month notice that Venus will move directly under the planet Saturn.
Binocular Sights (7 to 10 power): A good month to start working on your Messier list.
Telescope Sights (60-100mm): This is the month to catch your last views of the planet Saturn as it slowly sets in the west. Check out Venus phases.
Dobsonian telescope (6 -8 inch): This is the month to catch your last views of the planet Saturn as it slowly sets in the west. Check out Venus phases.
See you next month!