Tonight – August 29, 2017 – use the moon to find the star Antares and the planet Saturn, the 6th planet outward from the sun. Antares is reddish, while Saturn is golden. You might be able to tell that Antares twinkles more fiercely than Saturn, which shines with a steadier light. If you have a dark sky, you can see that Saturn now shines in front of the constellation Ophiuchus, sometimes called the forgotten zodiac constellation. Find out more about Ophiuchus here.
When you gaze at Saturn in the sky tonight, think about the indomitable Cassini spacecraft, which has been orbiting the ringed planet since 2004. It’s now about to run out of fuel and soon will be sent on a trajectory into the planet’s cloudtops, thereby ending its mission. That’ll happen in mid-September.
Meanwhile, Cassini’s last few orbits around Saturn have been spectacular, as the spacecraft has been diving between the outer atmosphere of Saturn and its innermost rings.
Saturn is the most distant world you can easily see with your unaided eye. Plus, you can view Saturn’s majestic rings with nothing more than a modest backyard telescope. All of the four outer planets (planets orbiting the sun outside the asteroid belt) – including Jupiter, Saturn, Uranus and Neptune – have a ring system of sorts. But Saturn’s rings are the most spectacular by leaps and bounds.
This year, in 2017, the north side of Saturn’s rings are maximally tilted toward Earth. So right now is a great time to dust off that telescope and gaze at Saturn, the crown jewel of the solar system.
Jupiter, Saturn, Uranus and Neptune are all gas giants (though Uranus and Neptune are sometimes referred to as ice giants). Overall, gas giant and ice giant planets have no solid surfaces. The smaller four inner planets with solid surfaces – Mercury, Venus, Earth and Mars – are called terrestrial or rocky planets.
There are no rings around any terrestrial solar system planet at present. Is there some reason why gas and ice giants have rings whereas terrestrial planets don’t? Cathy Jordon says at Cornell University’s Ask an Astronomer site:
It turns out that all of the planets, Earth included, did have rings at one time. The thing is, these rings were unstable and the material was either lost to space or collected into the satellites of these planets. The difference between the terrestrial and giant planets is the giant planets have the gravity to capture and hold onto a large satellite system, and these satellite systems are the source of the ring material.
It’s thought that Mars’ inner moon Phobos might break up and form a ring around Mars, some 50 million years from now. That’s because this moon is below the synchronous orbit radius – the distance at which the moon orbits Mars in the same time period that Mars rotates upon its axis. Because Phobos’ orbit is unstable, this moon is slowly but surely plunging toward its day of reckoning.
Bottom line: Let the moon on August 29, 2017 guide you to the planet Saturn, and if you have a telescope, use it to get an eyeful of Saturn’s marvelous rings.