Saturn in Sagittarius

July 19, 2013 NASA’s Cassini Spacecraft photographed a remarkable eclipse of the sun by Saturn. Giovanni Domenico Cassini discovered the ring structure of this formidable giant planet in 1675. Photo courtesy of NASA

If our “star” of July’s night sky was Jupiter in last month’s Backyard Astronomer, then this month’s best supporting planet must certainly be Saturn. Steadily shining a mellow yellow color, The God of Agriculture follows behind Jupiter, slowly rising higher in the southeast sky at dusk these August nights.

If you are reading this hot off the press (Friday, August 9), try going out tonight and spotting the waxing Moon at dusk—she will be very near Jupiter. On the night of Saturday, August 10, the Moon will be halfway between Jupiter and Saturn, and on Sunday night, August 11, the Moon will be near Saturn. This is an easy way to find these giant planets and start following them as they slowly move across the summer night sky.

As an added attraction to the Jupiter and Saturn show, the Perseid meteor shower will build up over the weekend and peak on August 14. The bright Moon will hamper seeing the smaller Perseids streak across the sky, but there still should be many bright ones visible, especially after midnight when the Earth faces the stream of rocks that burn up entering our atmosphere. Jupiter and Saturn are in the sky all night on these warm, short nights.

The Ringed Planet shines very brightly this summer for two reasons. The first, you might guess from last month’s exploration of Jupiter: Saturn, Jupiter and the Earth are all on the same side of our solar system during our current yearly lap around the Sun.

The other reason Saturn is so bright this summer is for a reason you might not suspect. Saturn, like our Earth, has an axis that is tilted to the plane of his orbit around the Sun. Since Saturn’s rings consist of a relatively thin, dense layer of small pieces of bright white water ice along Saturn’s equator, the plane of the rings appears at different angles to Earth as we both dance around the Sun. The rings vary from a tilt of 27 degrees from our vantage point on Earth, to being completely edge-on at 0 degrees and invisible in small telescopes.

When the rings are near their maximum tilt in relation to us, Saturn shines much brighter in the sky than when the rings are edge-on and invisible.

The rings were last at maximum tilt of 27 degrees in October of 2017 showing their north side. Currently the rings appear to us at about 25 degrees. The apparent angle will continue to decrease until they will be edge-on in the year 2025. After that, we will start to see the south side of the rings.

The rings are divided by concentric lanes at irregular intervals. The most prominent of these gaps is known as the Cassini Division, named after the astronomer who first noticed the different rings in 1675, Giovanni Domenico Cassini.

Cassini’s Division is another feature that the backyard astronomer can enjoy with a small telescope. When observing the rings, the careful observer will notice the shadow Saturn casts to one side or the other of the far side of the rings as they pass behind the giant globe. Once you spot this in a telescope, the planet really pops out in 3D. When Galileo first pointed his crude 20-power telescope at Saturn in 1613, he couldn’t quite resolve the ring structure. He described the rings as the “ears” of Saturn.

Saturn has 62 known moons, 53 of which are named. The gravity of some of the moons—mostly Pandora and Prometheus—act as a force to keep the rings confined to their narrow plane.

The largest moon of Saturn is Titan—second only to Jupiter’s Ganymede of all the natural satellites in our solar system. Titan is larger than the planet Mercury and is the only moon in the solar system with a dense atmosphere. This large, planet-like moon is easily visible in a small telescope; a constant companion to one of the most wondrous sights for the backyard astronomer.

Like Jupiter, Saturn rotates very fast: a day on Saturn is about 10 and a half hours long. Also, like Jupiter, this causes a noticeable bulge around the equator and spins Saturn’s dense atmosphere into bands along the lines of latitude.

Chart by Eric Fitzgerald

Saturn is currently in the constellation of Sagittarius (see chart). When we are looking into the summer constellation of Sagittarius, we are looking into the heart of our galaxy, The Milky Way. Perhaps this summer you might find yourself up in the mountains away from the city. Take some time on a moonless night later in the month to enjoy the summer Milky Way.

We will explore some of the deep sky wonders of Scorpius and Sagittarius in future installments of The Backyard Astronomer.


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