These next several evenings – August 27, 28 and 29, 2020 – watch for the waxing gibbous moon to pass by the two biggest planets in our solar system, Jupiter and Saturn. Both worlds are gas giants. Jupiter is bigger and by far the brighter planet, shining some eight times more brilliantly than Saturn. Yet the famous planet of the rings shines as brightly as the brightest stars. Just look up on these nights! You can’t miss these bright worlds near the moon.
And notice how close together in the sky these two planets are. That’s because they’re headed for a great conjunction later this year. If you find them in the coming evenings, you’ll enjoy watching them for the remainder of 2020.
Jupiter ranks as the fourth-brightest celestial body to light up the heavens, after the sun, moon and the planet Venus. But there’s no way to mistake Venus for Jupiter, or vice versa, because – at present – Jupiter rules the evening sky while Venus is up at dawn. Around the world, in the wee hours of the morning, Jupiter sets in the southwest at roughly the same time that Venus rises in the northeast.
Meanwhile, Saturn – the most distant word that you can see easily with the unaided eye – is twice as bright as the 1st-magnitude star Antares, Heart of the Scorpion. You might have seen the moon in the vicinity of Antares a few days ago.
There are two reasons for Jupiter and Saturn’s brilliance. First of all, these planets are huge; and, secondly, they have a high albedo (reflectivity). Jupiter has 1,331 times the volume of Earth, whereas Saturn has 764 times the Earth’s volume. Jupiter reflects about 52% of the incoming sunlight, while Saturn reflects about 47%. In contrast, our moon only reflects about 12% of the incoming sunlight.
Unlike the stars, which shine by their own light, Jupiter and Saturn shine by reflecting the light of the sun. Despite Jupiter residing 4.4 astronomical units (AU) from Earth and Saturn at 9.2 AU from Earth right now, these planets still stand out in Earth’s sky. By the way, one AU = one sun/Earth distance.
Saturn’s brilliance also depends the tilt of its majestic rings. You need a telescope to view the rings, but their inclination affects the planet’s brightness as seen from Earth even when looking with the eye alone. Saturn appears brightest when the rings are inclined at a maximum of 27 degrees toward Earth, and dimmest when the rings appear edge-on (0 degrees).
Presently, the rings are inclined at a little more than 21 degrees, adding to Saturn’s overall brilliance.
Bottom line: On August 27, 28 and 29, 2020, use the moon to locate the two largest planets of the solar system, Jupiter and Saturn.