With Venus approaching inferior conjunction, don't miss the chance to see one of the thinnest Venus crescents of your life.
On a recent sunny afternoon, I trained an old C-8 on Venus. But before looking in the eyepiece, I checked the 6×30 finderscope and easily saw the planet as a sharp, white crescent against a blue sky. It was so obvious a 3× finder would have nailed it ... if a guy could find a 3× finder. At the time, Venus spanned 58″ — nearly an arcminute — with only 2.6% of the planet illuminated.
While that crescent shimmered in its thinness, it will appear thinner and bigger yet on March 24–25, when the planet is at inferior conjunction with the Sun. On those days, Venus will maximize to 59.3″ with only 1.0% of the planet illuminated, equivalent to seeing a 19-hour-old crescent Moon.
Close attention to the crescent should reveal cusp extensions that can poke beyond the semi-circle to reach all the way around the dark limb. They're caused by sunlight scattered through Venus's upper atmosphere and appear faint and evanescent. Take a few minutes and use averted vision to see how far you can trace them.
If you have a Go To mount on your telescope, finding Venus in the daytime is easy business. Type in the coordinates, the scope slews, and bingo. For the rest of us who want to see the slenderest of Venusian crescents now through conjunction, you can manually offset from the Sun, the method we'll explore here.
We've all been warned never to stare directly at the Sun, especially when using a telescope. Since Venus lurks dangerously close to our star in the coming week (between 8.5° and 10.5° separation), you must take every precaution to protect your eyes when you seek it out. Never point your telescope directly at the Sun unless it's capped with a safe solar filter. Even a split second of magnified and concentrated sunlight will permanently damage your retinas. In addition, don't forget to cover your finderscope or that little beam of focused light could burn a hole in your shirt when you're not paying attention.
To use the offset method, you'll be putting those setting circles on your telescope to use. First, find the R.A. and Declination of the Sun and Venus for the time you plan to observe. Those positions are available live online at TheSkyLive or you can dial them up quickly with a free sky charting program such as Cartes du Ciel or Stellarium. Clicking on Venus and the Sun will bring up a live readout of their R.A. and Dec.
Now subtract the difference in their positions to obtain the correct offsets. For example, on March 22nd at 1 p.m. EDT:
- Sun = R.A. 0h 8′, Dec. +0° 53′, which we'll round to +1°
- Venus = 0h 11′, +10.5°
Since Venus is both north and east of the Sun at the moment, subtract the Sun's position from that of Venus to obtain an offset of 3′ east and 9.5° north of the Sun. Post-inferior conjunction, when the planet is west and south of the Sun, subtract Venus's position from the Sun's.
If your telescope is permanently mounted, you can apply the offsets straightaway, but if you're using a portable scope, point the polar axis north with the help of a compass while making sure the mount is level. Without the help of Polaris, your polar alignment won't be precise, but I've found in practice that "close enough" will often suffice. For greater precision, polar align the night before and leave your scope in place.
Place a safe solar filter over the telescope's objective, find the Sun, and then focus and center it in the eyepiece. Now, loosen the RA lock and carefully offset the right ascension 3′ east using your RA setting circle, then re-lock. Do the same with declination, pointing the telescope 9.5° north of the Sun. If your polar alignment is reasonably good, when you remove the solar filter and look through the eyepiece, you should see Venus staring back at you from a blue sky.
Don't see it? Nudge your scope a little this way and that to bring the planet into view. Caution! If you sense you're approaching the Sun too closely (brightening field of view), stop and retry your offsets.
If you're still getting nowhere, you can also use your finderscope with its wider field of view to find Venus, provided the field isn't so wide as to include the Sun. After doing the offsets, remove the cover and hold a piece of white paper in back of the finder. If you see bright sunlight shining on the paper, cover the finder back up. But if the Sun is out of the field, you can cautiously proceed to center Venus in the crosshairs.
It goes without saying that as Venus moves farther in apparent distance from the Sun, it becomes easier and less stressful to find. Using a solar filter and solar offsets, I've been able to spy Venus, Mercury, and Jupiter in the middle of the day on many occasions, both alone and paired in close daylight conjunctions. With caution, you can become a planetary observer both night ... and day.