December 21, 2020
See the two largest planets right next to each other as seen in our sky!
This conjunction will be a very nice holiday treat! Our two largest planets, Jupiter and Saturn, will be seen so close together in the sky that it will be a challenge to see them as separate objects with the unaided eye. Please look at the three images to the right.
Looking at the widest view, we see the horizon stretch across from south to west, centered on the south-west. You’ll see the first-quarter Moon toward the south. Also to the south, but much higher, you’ll see the red planet Mars. At twilight, around 5:30 p.m., you’ll see a very bright object about 20° above the south-west horizon. It will look like one object, but will be Jupiter and Saturn! This is the time to start looking. As darkness approaches, we witness the rotation of the Earth. We are rotating so that we are more fully immersed in the shadow of the Earth. Another effect is that the Earth is rotating away from our view of these two planets. Thus, they are seen to look like they are sinking lower in the sky. Waiting until later may mean these planets may be blocked by nearby trees, houses or mountains. Darkness comes at about 6:30 p.m. At 7:36 p.m., the Earth has rotated enough for the planets to be seen going below the true horizon. So, you have a small time window to see this conjunction.
The weather is another factor to consider. If you know the weather will be poor on the 21st, look the days before or after. The two planets won’t be as close, but will still be impressive.
If you have binoculars, the second image shows what you’ll see. The outer red circle represents a 4° field of view. This is similar to the view of most binoculars. You should be able to resolve the two planets now. They are at 6’18” separation. This means that they are 6 arcminutes, 18 arcseconds apart. You may ask yourself, what does that mean? If you trace out a circle all around you, like a hula hoop, that is 360 degrees (360°). That 4° binocular view is 4/360th = 1/90th of a full circle. A very small amount. Your index fingertip at arm’s length is about 1° wide. If the minutes and seconds part reminds you of a clock, it should. How many minutes are in an hour? 60. In mathematics, there are 60 arcminutes in one degree. And, there are 60 arcseconds in one arcminute. Our planets then, are seen about 1/10th the width of your fingertip apart!
The third image shows two telescope views of Jupiter and Saturn. The left image represents a common view of most smaller telescopes. At 91x, the two planets are easily viewed as well as their largest moons. The very unusual part is that they are seen in the same view! The right image is at 369x, an image from larger or better scopes. Both planets are still in the same view! This image, though, labels the bright moons orbiting each planet. Be aware that both of these views are with a refractor, a telescope with a lens in the front that gathers the light. The image is mirrored left-right. A reflector telescope that has a mirror in the back that gathers the light will provide a correct image, but upside-down.
Jupiter and Saturn are both gas giant planets. They are part of the Jovian planets that also include ice giants Uranus and Neptune. Jupiter and Saturn are so large, that they have an enormous atmosphere, made of gasses like hydrogen, ammonia, methane and much more. 1,000 Earths could fit inside Jupiter. 850 Earths for Saturn. Saturn, though, has those fabulous rings. The rings are made of small rocks and mostly ice. The rings are also larger than Jupiter. Binoculars will not clearly resolve the planets to the point of seeing the rings of Saturn. Saturn will be seen as not quite round, though. A telescope will easily show the rings of Saturn and should show the dark and light banding of clouds on the outer cloud surface of Jupiter.
Hopefully the weather will cooperate and you’ll see this very nice conjunction in your own back yard!