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 April 26th. For April, your best viewing nights will be from April 16th to the 30th. The Star chart below is set for Florence, SC on April 15th at 9 pm. Note: A table of contents for earlier columns is located in the April 2016 column.
The “Green Flash”
The Green Flash is a rare solar phenomenon that few people have ever seen. I have yet to see this event although I looked for it over many years. Regular visitors to ScienceSouth know that we have a great rocket scientist in our group known as Mister Fred. Recently Mister Fred casually mentioned to me that he has seen the Green Flash. I was shocked, and at that point, I realized that I had never discussed this event in my column.
The Green Flash is a rare event that occurs during sunset. Just as the top of the Sun is about to disappear below the horizon, a green flash of light is seen on the top of the Sun. I have been told that this can also occur during sunrise, but almost all reports of this event occurred during sunset. This Green Flash lasts only one to two seconds; which also adds to the rare chance that you might see it. The best chance you have of seeing this event is your knowledge that it can happen. Therefore, once you are aware of this phenomenon, from this day forward, you will now watch for it whenever you view a sunset.
The Green Flash requires a sharp horizon. Therefore almost every report of this event has been when the Sun is setting over the ocean, as shown in the images above and below. Therefore, within driving distance, either the west coast of Florida, or a cruise ship would give you your best opportunity. The amazing image below was taken on the beach at Clearwater, Florida.
Let me clarify the sunset favored over the sunrise. If you are looking for a green flash as the Sun is setting in the west, your eye can clearly follow the disk of the sun disappearing into the ocean, and anticipate the exact moment that the green flash might appear. However, on the South Carolina coast watching the sunrise, you have no clear disk to follow, because it is below the horizon, therefore, you have to approximate where or when it might appear, lowering your chance of success.
The science behind this effect is simply light refraction. As the Sun sets or rises, the atmosphere is acting as a prism refracting light. When the Sun sets, the white light passes through a longer portion of the atmosphere than when is high in the sky. We have a blue sky because the atmosphere scatters blue, indigo and violet light. During a sunset, you essentially remove all of the BIV from the rainbow ROYGBIV, leaving ROYG. The ROY overpowers your eyes, and we see a red sun setting, however, the atmosphere is still refracting/separating the colors. So the blues are already gone, the red Sun then goes below the horizon, and the only color left is green! This sounds simple in theory, but it takes extremely special atmospheric conditions to see this rare event.
There is no bright star on the southern horizon!
Periodically people will call ScienceSouth with a question about astronomy. In March someone called and asked what the bright star near the southern horizon was after sunset. This question was then directed to me. I asked if the person was looking at Venus, which is not very close to the horizon, and in the west, not the south. The only other bright object that night was the bright star Sirius, in the south, but almost 40 degrees above the horizon. The woman who called was aware of both the location of Venus and Sirius that night, and stated that the unknown bright star was almost touching the southern horizon. Based on my lifetime of observing the night sky, my answer was, “there is no bright star on the southern horizon.”
Now a good scientist must train them self to be humble, and to always keep an open mind. Therefore, I turned on my astronomy computer program and looked at the southern horizon just after sunset, and there was a nice bright star only 4 degrees above the horizon! Impossible, but there it was, the star Canopus in the southern constellation of Carina. The caller was right, and I was wrong. It so happens that my lifetime of observing took place above 41 degrees north latitude. At this latitude, Canopus would not be visible. In addition, my viewing sites since I have been in South Carolina have not had a clear view of the southern horizon. So I said there was no bright star on the southern horizon because I never had a place to see it.
View of Canopus on March 15th.
The star Canopus is visible from South Carolina every March, if you have a clear view of the southern horizon. Note: There is a small chance that you will be able to see it early this month.
Naked Eye Sights: Maybe Canopus on the southern horizon early in April. The winter constellations are still visible but not for long.
Binocular Sights (7 to 10 power): Comet 41P/Tuttle–Giacobini–Kresák, mentioned last month, is moving downward through the constellation Draco in the first two weeks of April.
Telescope Sights (60-100mm): Jupiter in the east.
Dobsonian telescope (6 -8 inch): Jupiter in the east.