What are those bright things in the sky?
Published on Jun 9, 2007 at 1:24 pm.
4 Comments.
Filed under skywatching.
Several years ago, someone called the campus to ask to whom he should report an UFO sighting. The campus operator kindly put the call through to me. Well, I asked the caller just what he’d seen. He said that he’d seen a big bright UFO hovering over Fort Worth, Texas, just after sunset. I asked him where he lived. He told the name of a Fort Worth Suburb to the northeast. That year, the planet Venus was in the southwestern sky at sunset. So, I told him that he’d probably seen the planet Venus. “Oh, no way!” he insisted. He went on to explain that he’d gone in to eat dinner with his wife, and when he came back out to look at the UFO, it had flown away. I explained that it had set. That was a surprise to him that the sky could move so fast. But, for people not familiar with the sky, a lot of things that they see up there can be a surprise. Venus is one of them. It can be the brightest object normally in the sky other than the Sun or Moon (on very rare occasions, a meteor can be brighter). Thus, it is no wonder that Venus is routinely reported as a UFO by casual observers who happen to look up. Back a few decades ago, a fighter pilot actually spotted Venus and chased it for a while before he reported the the UFO that he was chasing was too high for him to reach!
Venus’ orbit brings it into view every two years, or so. Venus for some time now has been the very bright object in the western sky at sunset. It is now a bit more to the northwest, and it is still very bright. Shining at magnitude -4.2, Venus is 13 times brighter than the brightest star, Sirius. Venus is currently about 66 million miles away. That is a pretty fair distance, but Venus is nearly the same size as the Earth, so it makes for an object about 23″ across in the telescope. The only other planet bigger than that in the telescope right now is Jupiter, at about 46″. (One arcsecond, “, is 1/3600 of a degree). Venus orbits the Sun closer to it than does Earth, so it is always seen in the sunward direction in the sky. But, sometimes it appears closer to the Sun, and sometimes farther away from the Sun. Tonight, it will be about as far from the Sun as it is going to get this time around, about 45° from the Sun. We call this Venus’ Greatest Eastern Elongation. Because of the varying angle between Earth, Venus, and the Sun, Venus appears to go through a set of phases similar to those of the Moon. In fact, when we look at Venus in the telescope during public star parties, people from the general public sometimes mistake it for the Moon. If you were to look at it in the telescope tonight, Venus would appear as half lit up by the Sun. Venus’ current distance from the Sun means that it will be up for a couple hours or so after sunset, so a lot of people will see it, and will likely report it as an UFO.
But, there is another bright thing in the sky tonight after sunset! In the opposite direction, towards the east, just after sunset a bright object will be rising. After an hour or so, it will be as high in the sky towards the east as Venus is towards the west. Shining with magnitude -2.6, this object will be dimmer than Venus, but still nearly double the brightness of Sirius. It is the planet Jupiter. Jupiter is the largest planet in the Solar System, nearly the size of 1000 Earths. It also has more mass than any other planet, almost 320 times the mass of Earth. It is over 400 million miles away from us right now, but it still appears as the second brightest object in the sky tonight other than Venus, and as the largest planet as seen in the telescope. Only a few days ago, on June 5, Jupiter was virtually directly opposite to the Sun, as seen from Earth. We call this orientation Opposition. That meant that it was rising just as the Sun was setting. Every day since then, it has risen nearly 4 minutes earlier than the day before.
With both of these very bright objects low on opposite sides of the sky, you can rest assured that some people are going to be thinking that they are objects flying in formation. I expect a flurry of UFO reports. You might wonder how often two of the brightest planets are up on opposite sides of the sky like this. Well, it isn’t as infrequent as you might think. It happens every few years. But, to notice it, you need to pay attention.
Oh, and Venus and Jupiter are not the only two planet that you can see these nights. Up and to the left of Venus is what looks like a triangle of fairly bright stars. The “star” on the lower right of the triangle is actually the planet Saturn. Compared with the other planets, Saturn is much dimmer, at magnitude 0.5. That still makes it the brightest of the triangle of three “stars” up and to the left of Venus. The stars are Regulus an Algieba, two stars of Leo.
So, go out and look for the planets tonight!
-Astroprof
A Ler…-- Rastos de Luz on June 11, 2007 at 8:59 am: 1
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Lab Lemming on June 13, 2007 at 7:03 am: 2
My star finder program (Stellarium) says that Venus is currently -4.2, and Jupiter is currently -2.15. For Saturn it says 1.2. Why does it give different numbers?
Also, Mercury was seeable in the first hald of the month.
Astroprof on June 13, 2007 at 3:59 pm: 3
Good question. I’ve noticed that different sources often give somewhat different values. Maybe it is the algorithm used. Or, perhaps they are defining different ranges of light (the entire visual spectrum or just the Johnson V range).
Marcianitos Verdes » ¿Qué son esas cosas que se ven en el cielo? on June 14, 2007 at 10:10 pm: 4
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