Slim and bright

Venus continues its spectacular evening show, growing ever brighter toward its peak brightness of -4.7 around the end of the month. By then it has waned to a crescent. That’s a bit non-obvious: when the moon is a crescent, it’s a lot fainter than a full moon. So why is Venus brightest in its crescent phase?

It has to do with their orbits. The moon is always about the same distance away, about 385,000 km or 239,000 miles (I’ve owned cars with more miles than that!), though it varies a little, from 362,600 km at perigee to 405,400 km at apogee.

When we look at the full moon, not only are we seeing the whole Earth-facing surface illuminated, but the central part of that light is reflecting straight up off the moon’s surface. When we look at a crescent moon, we’re seeing light that’s near the moon’s sunrise or sunset point – dimmer and more spread out than the concentrated light of noon – and in addition we’re seeing less of it.

Venus, in contrast, varies its distance from us immensely. We can’t see Venus when it’s “full”, because it’s on the other side of the sun from us and lost in the sun’s glow. It’ll be more than a year before it’s full next, in April of 2013. But if we could see it when it’s full, Venus would be a distant 1.7 AU from us – about 170 million miles – and its disk is a tiny 9.9 arcseconds – about the size of Mars this month.

In contrast, when we look at the crescent Venus around the end of this month, although we’re only seeing about 28% of its surface illuminated, and that only with glancing twilight rays, it’s much closer to us – less than half an AU, or about 45 million miles. So its disk extends a huge 37 arcseconds, bigger than Jupiter this month.

Of course, eventually, as Venus pulls between us and the sun, its crescent gets so slim that even expanding size can’t compensate. So peak brightness happens when those two curves cross, when the disk is somewhere around 27% illuminated, as happens at the end of this month and the beginning of May. (Exactly when? Good question. The RASC handbook lists “greatest illuminated extent” on April 30, but PyEphem and XEphem say Venus is actually brighter from May 3-8 ... and when it emerges from the sun’s glare and moves into the morning sky in June, it’ll be slightly brighter still, peaking at magnitude -4.8 in the first week of July.)

Mars is also visible in April evening skies, but it’s past its opposition and already shrinking. As I write this, in March, I haven’t heard many reports of good Mars observations – the weather hasn’t been cooperative in giving us those steady skies we need to see detail during such a distant opposition. But keep trying – you never know when you’re going to get lucky with the weather.

During the first week of the month, at around 9pm, you’ll be looking at Mare Acidalium, Sinus Sabaeus and Sinus Meridiani. By the middle of the month, Syrtis Major dominates the center part of the disk, with Hellas looking like a polar cap in the south, and more subtle features like Nilosyrtis and Utopia ringing the north pole. A week later, you’ll be looking at Cimmerium and Tyrrhenum in the south, Mare Boreum ringing the north pole, and very subtle features like Cerberus and Trivium challenging you to pick them out of the washed-out center of the disk. Then it gets even more subtle for the last weekend of the month, with Sirenum rotating in far to the southwest, Boreum on the northeast, and very few discernible features at all in the center of the disk.

If you keep watch on Mars regularly during the month, pay attention to its phase over the course of the month: it’ll go from nearly full to noticeably gibbous.

Saturn is high in the sky and hits opposition on April 15, with rings tilted at 13 degrees to us. On the other end of the sky, Jupiter disappears into the sun’s evening glow during April.

Mercury is in the dawn sky. Early rising Mercury watchers might be particularly interested in it around April 22, when it acts as a nice pointer to Uranus, two degrees above and to the left of Mercury.

Uranus probably wouldn’t be bright enough to see in the twilight, but with Mercury to point the way, it might be an unusual morning sight.

Neptune, too, is in the morning sky, but it doesn’t have anything nearby to point the way, so you’re better off waiting a few months.

Pluto rises around midnight, so if you want to catch it you’ll have to stay up late – or get up very early.