Saturnian moon transits, and other subtle events

In February, Venus continues its spectacular and high pass. It's been putting on a great show this year, and as the month progresses, Venus will draw closer to the sun while expanding into a large, thin crescent. It's always fun to watch Venus' phases. At a public star party, if you point a telescope toward a crescent Venus, you'll always get a few people who ask "Is that the moon?" even if the moon is clearly hanging right there, gibbous or nearly full, on the other side of the sky, or isn't there at all. I always wonder whether they think Earth actually has two moons (they just don't tell you that if you don't have a telescope, so you don't get disappointed), or maybe that telescopes can look backward or forward in time.

Saturn continues to move toward its early March opposition. Its rings have already opened from truly edge-on, but their angle of about a degree and half still isn't much and may well still look edge-on in most telescope views. Keep an eye on it as the rings widen throughout this month and the following months: when do they become easily visible?

It should also be fun to see Saturn's moons without any distraction from the rings. With Saturn's ring plane pointing so nearly at the sun (the sun-ring angle is less than three degrees this month), there are lots of transits of Saturn's moons and their shadows. I've never seen a shadow transit on Saturn; have you? Saturn's moons (except Titan) are quite small and faint compared to Jupiter's Galilean satellites; a Saturnian moon transit is a subtle, challenging event. They've been imaged in amateur telescopes, but I haven't heard of anyone seeing one visually. But maybe that's because no one has tried!

Use a planetarium program to see when good candidates might occur. For instance, XEphem shows me that on Saturday Feb 7 at around 10 p.m., both Tethys and Dione and their shadows will be in transit. Saturn isn't very high then, only about 23 degrees, so it's a balance between waiting for it to rise into steadier air and the ends of the transits (by 11pm, Saturn will be up to 35 degrees but Tethys and both shadows will already be finished transiting).

On Saturday the 21st (a third quarter Saturday), Tethys and its shadow will again be transiting, but by then Saturn will be higher in the sky and into steadier air. The transit starts around 8:30, though Tethys will already be in transit against the rings before then. (Will the transit against the rings be visible? I doubt it, but it doesn't hurt to look!) The shadow exits the planet's globe about 11 p.m., with Saturn a respectable 45 degrees up.

If you can stay up until 3 a.m. on the morning of the 9th (unfortunately that’s also a full moon evening), you can see the beginning of a Rhea transit (Rhea is Saturn’s second largest moon), with tiny Mimas and its shadow beginning a transit an hour or so earlier. Enceladus follows on Rhea’s heels.

But the real prize comes in the wee hours of Tuesday, Feb 24, with a very rare Titan transit. It’s a new moon, so the sky will be dark. Titan’s shadow begins its transit at a hair before 2 a.m., and the planet itself touches Saturn’s limb an hour and a half later. Titan is big enough that its shadow should be easy to see. Titan transits only happen when the planet’s ring (and moon) plane are close to edge-on, as they are now. There will be another one on Mar 12 at 4 a.m., then by the end of March Saturn’s tilt has grown too great. Later in the year when the ring angle closes up again, Saturn will be too close to the sun for us to see anything.

Anyway, these aren't the only transits, just a couple that I noticed were happening on weekend evenings. If you're interested, fire up some software that shows moon transits and check out times when you think you might be up late. I expect these transits will be tough to see unless you have very steady skies and first-rate optics, but don't let that stop you from trying. Remember: you won't see what you don't look for!

Early morning risers get a great show in February as well. From the 20th through the first few days of March, Mercury, pulling away from the sun, shoots past first Jupiter and and then Mars. First Mars passes Jupiter, on the 17th; then on the 24th, Mercury passes Jupiter; finally, on March 2, Mercury passes Mars. Each of these encounters involves a separation of less than a degree and should be a nice view in any telescope.

That's the good news. The bad news is that this all happens quite low to the horizon, so you'll want a decent eastern horizon to catch it. Jupiter is bright enough that it should remain visible even as the sky brightens (though it does get more difficult to find), but Mercury and Mars will probably fade out fairly quickly.

Another show – sort of – for morning people is a penumbral lunar eclipse on the 9th. A penumbral eclipse isn't much to see, unfortunately; it's so subtle that you might not even notice one was happening if you didn't already know. But now you do know, so if you find yourself up in the wee hours of February 8, take a look and see what you see. The moon starts to enter the penumbra at 4:38 a.m.; by the time it sets, at 7:02, it's past mid-eclipse but still almost entirely within the penumbra. Official sunrise is one minute later, but the sky will already be fairly bright by that point, making the eclipse even harder to see.

Ceres is at opposition on Feb 25, in Leo, hanging over the lion's haunches. That's a curiously empty part of Leo: the nearest deep-sky object is NGC3344, about 3.5 degrees directly above Ceres. This is an unusually close opposition (a little over 1.5AU) and it won't get closer than this again until the year 4164.

The outer planets don't put on much of a show this month. Uranus is barely visible in the early evening; Neptune and Pluto remain hidden in the Sun's glare.