Observing Venus Near Inferior Conjunction: By Guest Host, Richard Nugent From Massachusetts

    Venus passes through inferior conjunction every 19 months and during the week prior to and after I love to observe her. Why? Because during inferior conjunction Venus is passing between the Earth and the Sun. Its angular diameter is large because it is closest to Earth and it offers a unique view of the planet: a razor-thin crescent! This month, on October 26, Venus will be a generous 6°20’ from the sun making this inferior conjunction particularly easy to observe. Her disk will be slightly larger than one arcminute and she will be 0.6% illuminated. So, how do we observe this!

    The region of the sky this close to the sun is a perilous place to be observing. Your telescope will be unfiltered so aiming the telescope is critical. You do not want to be sweeping in this part of the sky!  In order to know where to look I use SkySafari Pro but any planetarium program will work. If you have a go-to or push-to telescope, carefully align the scope and let the computer guide you to Venus. If you are using good, old-fashioned setting circles make sure your mount is polar aligned, set the R.A. circle to the proper sidereal time, get the right ascension and declination for Venus and go to that spot.  I live in the alt-az world so I get that info from my program and then use the phone’s compass and tilt meter to get to the correct spot. I find the tilt meter to be more accurate than the compass so I get close then carefully…I mean CAREFULLY sweep in azimuth until I spot Venus. I typically use a 10-inch, f/5 dob with an 80mm Finder. Today, Venus was easily visible as a crescent in the finder. Once it’s in the finder you’re home free!

   One important tip is to pre-focus your eyepiece. If Venus is out of focus it’s crescent will smear out and blend into the bright background. I start with a low power eyepiece and graduate to my 16mm Nagler. This gives about 75x with a generous amount of sky around Venus. The seeing is usually terrible during the day but I find that an aperture mask is particularly useful in reducing the turbulence. Today, I ran the scope at 60mm. [f/19.9 with a 0.8mm exit pupil] The crescent was magnificent! During moments of steadier seeing I thought I could see the entire limb of Venus but that just might have been my brain connecting the cusps to complete the circle. I’ll look a little closer towards inferior conjunction when the effect should be greatest.

    I’m really a visual astronomer but sometimes I can’t resist the urge to snap a picture. The image here was taken by holding my iPhone (8 Plus) up to the eyepiece. I use the camera zoom to focus the telescope then zoom out a little and shoot bursts of images. I select the best shots, crop them, and adjust the exposure if necessary.

   We only get a couple of weeks every 19 months to observe Venus this way so I use every clear opportunity to make observations. The next inferior conjunction of Venus wont be until June 3, 2020 but Venus will be too close to the sun to view. In that case, I’ll observe Venus up until a few days before then wait a few days after the actual conjunction. My strict limit is 3-4 degrees away from the solar limb. As I said…perilous!

    I’d encourage you to try to see Venus this week. Have fun but please be careful!   RN 


Explore posts in the same categories: Uncategorized

%d bloggers like this: