See an Ultrathin Venus Crescent

With Venus approaching inferior conjunction, don't miss the chance to see one of the thinnest Venus crescents of your life.

Edge of Venus

Cellphone photo of Venus through an 8-inch SCT on March 18th, when the planet was 2.6% illuminated and stood 13° east of the Sun.
Bob King

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.

Pointy Planet

Venus as a razor thin crescent on March 21, 2017. Faint cusp extensions are visible on both sides of the crescent. The planet was 1.8% illuminated at the time and a little more than 4 days before inferior conjunction.
Shahrin Ahmad

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.

A Waning, Growing Venus

Raffaello Lena in Rome has been capturing Venus as it wanes in phase and enlarges in size, using a 7-inch Maksutov-Cassegrain scope. He plans to continue right through conjunction.

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.

Speedy Venus First East Then West of the Sun

On March 25th, Venus reaches inferior conjunction when it passes between the Earth and Sun. Because of its orbital inclination, the planet passes about 8° due north of the Sun at conjunction. Shortly before, during, and after conjunction, Venus will appear as a wire-thin crescent. The planet moves west of the Sun after conjunction and will reappear low in the eastern sky before sunrise by the first week of April.
Wikipedia, with additions by the author

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°
Safe Shroud

Once you've aligned you've polar aligned your telescope, cover your finderscope before proceeding to point the telescope sunward.
Bob King

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.

Solar Filter Safety

Place a safe solar filter on your telescope for centering on the Sun, then do your offsets. Remove the solar filter after you've directed your scope away from the Sun to find Venus.
Bob King

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.

Offsetting to Venus

Move the telescope the desired amount of right ascension (RA) and declination (Dec.) from the Sun to arrive at Venus. This RA circle has a calibration mark, making offsets even easier.
Bob King

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.

Clockwise Crescent

Venus glides north and west of the Sun in the next week. As it moves from evening to the morning sky, the crescent rotates clockwise from bottom to left around the planet's limb.
Illustration: Bob King, Source: Stellarium

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.

13 thoughts on “See an Ultrathin Venus Crescent

  1. Jakob

    At sunset today I saw Venus with no optical aid which surprised me. Never before have I seen our sister planet so close to inferior conjunction. Hope to see it in seven hours time! J

  2. Tom-Reiland

    Bob, I was able to view Venus Wednesday evening through my 10 X 50 binocs and the 21″ Reflector and 5″ Refractor at Wagman Observatory. After sunset, I was able to see it nude-eye (naked sounds so dirty) : ) I was in no shape to try to see it again on Thursday morning, but two of my friends in the Amateur Astronomers Association of Pittsburgh observed in the evening and the following morning. I was somewhat worn down after observing from 7:40 PM to 4:40 AM with a few breaks for food and to warm up. It was worth it. I observed 7 more H objects and confirmed several others that I wasn’t sure of my previous observations. 19 more to go. I’ll let you know when I finish. I’ve heard that one observer, maybe two, have accomplished this feat other than William and John Herschel. Have you heard of anyone? Thanks again, for the heads up on another fun observing project.

      1. Tom-Reiland

        Bob, I heard about the two or three observers from Terry Trees, the AAAP Rep to the AL. He said that several observers claimed to have observed all the H objects. One said that he sketched them all and another photographed them. Also, at least one of them used a Goto system to find them. That does not impress me at all. It’s not following in William Herschel’s footsteps. All my observing is done by Starhopping. Without Starhopping and learning various patterns of stars, I would not have discovered Reiland’s Cluster in Cepheus in 1985 from my backyard with my homemade 8″ Reflector. Nor would I have been the only visual discoverer of the SN in M51 in 2011.
        One of my mentors and long time friend, Jim Mullaney, said that he hasn’t heard about anyone viewing all 2,500 to 2,520 minus 4 non-existent objects, depending on who’s research one wants to believe. I’m working on an article for S & T that I’ll submit when I complete observing all of these objects. Jim told me to contact Peter Tyson. I’ll keep you informed. Thanks again for all you do to promote observational Astronomy.

    1. super

      I am writing from Uganda and I am interested in sky and Telescope but the problem we have here is the poor quality of Chinese things which are flooding our markets here. some few weeks ago I bought a Chinese Binocular, its not all that I need some advise. Thanks Peter.

  3. Joe StieberJoe Stieber

    I too saw Venus on 22-March-2017. I initially picked it up at 1:07 pm EDT from my NJ home with 10×50 binoculars by moving 9.5 deg straight up from the sun (which was blocked by my neighbor’s roof). I did not see it with unaided eyes as I have done a couple of times in daylight the past month. I found it again with my 80 mm refractor at 6:50 pm from Belleplain State Forest in NJ (I was there to assist with a college student astronomical field trip). I subsequently saw it with 10×50 binoculars, but not with unaided eyes. Because of the six-degree tree line in the west, Venus was still in the whitish glare of low altitude scattered sunlight. Venus was lost in the trees before sunset at 7:14 pm. The following morning, 23-March-2017, I spotted Venus with the 10x50s from Pennsauken, NJ, at 6:40 am when it was a little more than 2 deg altitude (sunrise would be at 6:58 am). I was then able to glimpse it with unaided eyes. In each case, the crescent was easily visible in the 10x50s and sublime with the 80 mm refractor. I’m hoping for a break in the weather tomorrow morning, 25-March-2017, so I can see Venus shortly after inferior conjunction, which is at 6:17 am EDT, the same time it rises here at 40N-75W.

  4. Anthony BarreiroAnthony Barreiro

    This morning, ten minutes before sunrise, from the back stairs of my home, I saw Venus about three degrees above the trees on the ridgeline of Potrero Hill in San Francisco. Two days after inferior conjunction. She was a breathtakingly thin crescent through 8×42 binoculars. I was somewhat morose until I saw Venus, and I’ve been happy and energized since.

    1. Bob KingBob King Post author

      Hi Anthony,
      That crescent put a smile on your face. Great sighting so soon after conjunction! It helps doesn’t it that Venus is significantly north of the Sun.

      1. Anthony BarreiroAnthony Barreiro

        Yeah. I was hoping to see Venus in the evening and morning, or morning and evening, of the same day last week, but cloudy and rainy weather, hilly topography, other commitments, and a bit of fatigue prevented me. I’ll have to settle for seeing Venus before sunset five days before conjunction and at sunrise two days after.

        1. Bob KingBob King Post author


          Looks like you’ll have to try again next apparition of the planet. That’s one thing about amateur astronomy – you get lots of second chances – as long as you’re patient.

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