Observing Saturn and Jupiter in 2021

Saturn and Jupiter will be well placed for observing in the second half of the summer and into the Fall. DDOD Astronomers Dr. Ian Shelton and Dr. Tuba Koktay recently obtained images of the two planets using their 10-inch telescope from Richmond Hill in Ontario. The first image shows Jupiter with its Great Red Spot visible. The second image taken two hours later clearly shows how quickly the planet rotates. The third image displayed at the same scale as the first two images shows how Saturn’s much greater distance makes it look much smaller than Jupiter in telescopes, even though the two planets are actually similar in size.

On the night of August 1, 2021, Saturn will be at “Opposition”, meaning that it is as close to exactly opposite (180 degrees away from) the Sun as its orbit and our view of it from here on Earth allows. Observationally, Saturn will rise near sunset, set near sunrise, and be at its highest point in the sky as it passes due south (“transits”) at local midnight. As seen from Richmond Hill (and for much of the GTA), Saturn will rise at 20:35 EDT, sets at 6:14 EDT, and transits at 1:25 EDT.

Being at opposition also means that Earth is passing at its closest point to Saturn as the two planets travel along their respective orbits around the Sun. On this night, Saturn will be only 1.3367 billion kilometres away from us. Ok, not exactly what most people would consider “close”. But this is when Saturn appears largest in our telescopes, though still only about 18.6 arcsec in diameter (~100 times smaller than the full Moon). That’s for the ball of the planet; the rings extend about 1.3x further outward from the planet, spanning 43 arcsec. But the rings are never seen fully face-on and are currently tilted by about 18 degrees from being seen edge-on, making the rings look only ~13 arcsec wide. With an adequate telescope (at least a 75 mm lens or mirror, operating at 100x or more) and good atmospheric “seeing” (stability), this allows the southern hemisphere of the planet to now be partly visible again after having been blocked from view when the rings were near their maximum tilt of 27 degrees four years ago. We are currently seeing mostly Saturn’s northern hemisphere. But don’t expect to see any details on the ball of the planet: Saturn rarely shows anything but subtle bands wrapping around the planet.

On the night of August 19, 2021, it will be Jupiter’s turn to be at Opposition. Being 19% physically larger and orbiting at half the distance that Saturn orbits from the Sun, Jupiter will display a disk 49.1 arc-sec in equatorial diameter, more than double the apparent size of Saturn. And during Oppositions, Jupiter appears 25% larger than its average size as viewed from the Earth. The planet will be 600 million kilometres away from us on this night. Unlike Saturn, a modest size (~75mm) telescope is enough to reveal the bands and belts that perpetually define the upper layers of Jupiter. And even a good pair of binoculars will reveal Jupiter’s four largest moons, rearranging themselves into a different configuration each night as they orbit around the planet in just a few days. Three of these are larger than our own Moon.

Jupiter is easy to find in the night sky, looking brighter than any star. Saturn may prove a bit more challenging because it is 15 times fainter; but it is still about as bright as the top ten brightest stars in the sky, and so reveals itself by looking out of place if you know your constellations. The first sky map shown below will help you find the two planets this summer. It should match the sky if you are looking at around 11 pm in mid-August. If you are looking earlier in either the month or in the night, the planets and surrounding stars will appear further towards the East (E); if you are looking later in the night or the month, look for the planets further towards the West (W). The second sky map shows the region centred on Saturn at the time of its opposition (at 1:25 EDT on August 2).

To learn more about Saturn and Jupiter, have a look at the following websites:

https://solarsystem.nasa.gov/planets/saturn/overview/

https://solarsystem.nasa.gov/planets/jupiter/overview/

Wishing everyone clear skies,

–Ian Shelton