Blue Ring Around Uranus

Jupiter reaches opposition on May 4 and is visible all night, though of course it’s best near midnight when it’s highest in the sky. Unfortunately, it’s quite far south and never gets as high as 40 degrees even at transit.

So this may not be the best year for seeing fantastic detail on Jupiter (unless you’re planning a trip to the southern hemisphere) -- but it’s exciting nonetheless, because of Jupiter’s small new red spot, the one I talked about in last month’s column. Maybe the rains will finally stop and we’ll get a chance to observe this new feature on Jupiter, even low in the sky, in some good spring weather.

Saturn and Mars are both fairly low in the west as night falls, with Saturn setting a bit later. They’re both catchable, but you’ll have a hard time seeing much detail.

Mercury and Venus are both low in the morning sky at the beginning of May, so you’ll have to work to see much of any of those planets this month. Venus remains there, but Mercury disappears after the first week, reappearing in the early evening sky as we move into June.

Uranus, in Ophiuchus, and Neptune, in Capricornus, are also available in the morning sky for dedicated observers, but you’ll get a better look at them if you wait a few months. Pluto is a bit ahead of them, rising before midnight and transiting at 3am, but Plutocrats will still to stay up late to catch the dim roughly 14th-magnitude speck.

Uranus has been in the news this week because of its recently discovered “blue ring”. The outer ring of Uranus was actually seen for the first time last year, but a new paper in April said that the ring was colored bright blue, just like Saturn’s E ring. Of course, these are very small, subtle rings, far to faint for us to see at all in a telescope from Earth. But it’s interesting to contemplate: what makes the rings blue?

Previous theories about Saturn’s blue E ring speculated that the ring, unlike Saturn’s brighter rings of tiny rock and ice particles, was made of gases erupted from plumes on Enceladus. But Uranus isn’t thought to have a moon similar to Enceladus (its outer ring is thought to be kept in line by a tiny moon named Mab, only 15 miles across). So some other explanation is in order.

The new explanation is that the blue rings are made only of the tiniest dust particles, less than a tenth of a micron in size. Some peculiarity of resonance with a moon keeps larger particles out of the ring. These tiny particles scatter blue light, just as the particles in our own planet’s sky do. The blue rings are blue for the same reason Earth’s sky is blue!

You can demonstrate scatter and how it affects the color of the sky by mixing small particles into a clear container of water, then shining a bright white light into the container. Try flour, silt from a local stream (should be easy to find after this season of heavy rains), or a small amount of milk to see what gives the best effect. If you get the particle size right, the blue light shining in will scatter, creating a “blue sky” in your container, while most of the red light will pass right through, mimicking a beautiful red sunset. What happens when you shine only monochromatic light, such as a red laser or a green laser, into your container?