From the March 2016 issue

Transit of Mercury

Erika Rix prepares astrosketchers for May 9th, when Mercury will transit the Sun.
By | Published: March 28, 2016 | Last updated on May 18, 2023
On May 9 at 11:12 UT, Mercury will begin its 7½-hour journey across the face of the Sun as seen from Earth’s vantage point. Observers in eastern North America will be treated to the entire transit while those farther west will need to wait until sunrise to join in.

Mercury’s disk will be very small, only 1/158th that of the Sun. So before you head out, you should familiarize yourself with its contact points along the solar limb, or edge, and its transit path. You can use ephemeris predictors like WinJUPOS. Developed as freeware by Grischa Hahn, WinJUPOS is a valuable planning tool that is dedicated to solar system objects. After entering the pertinent data for your observing location and the viewing orientation of your telescope, you can view a graphic of Mercury’s predicted position relative to the Sun.

Sally Russell sketched Mercury transit
Sally Russell sketched this Mercury transit on May 7, 2003, from 04:47 to 10:35 UT using a 105mm f/6 refractor on an alt-az mount fitted with a white light solar filter and a 12.5mm eyepiece for a magnification of 48x. She used heavy weight cartridge paper and a B graphite pencil for the sketch. She created the black dots representing Mercury during the observation. Afterward, she added the small black circles as a comparison to represent the actual transit line.
To safely observe the event, you’ll need to use either a solar filter that fits over the front of your telescope or eyepiece projection (and a solar filter is best). Keep in mind that first and fourth contacts, before and after Mercury’s disk crosses the Sun’s, will be viewable only with a narrowband solar filter, and then only if Mercury happens to pass in front of a solar prominence. But remember that a narrowband filter also will reveal the mottled texture of the Sun’s chromosphere and, due to its size, Mercury will be harder to track against such a background. A white light filter, on the other hand, shows the Sun’s uncluttered photosphere, making tracking the planet easier.

Ideally, your sketch of the Sun should be completed just before the transit begins. I draw a 6-inch circle to represent the solar disk; large sketching areas allow room for finer details to be added. I also set an alarm to go off 10 minutes before the transit begins so that I don’t lose track of time. Keep extra pencils on hand, sharpened and ready for use.

Jeremy Perez sketched Mercury transit
Jeremy Perez sketched his Mercury transit on November 8, 2006, from 19:33 to 23:54 UT while using a 6-inch, f/8 equatorially mounted telescope fitted with a white light filter and a 25mm eyepiece for a magnification of 48x. He used an HB pencil and blending stump on plain 20-pound white bond paper. He added the limb darkening later with Photoshop. During the transit, he used the sketch to show people where to look on the disk for Mercury and which spots were sunspots instead of the planet.
Several minutes before first contact, focus your attention on the area of solar limb where you expect Mercury to emerge. It will pass quickly, but try to capture its disk taking a bite from the Sun’s edge. Once contact is made, use a pencil to draw Mercury on your sketch and plot its course along with the corresponding times for each addition. Sunspots and other visible solar features can be used as reference points for the planet’s size and positions. For those of you who will miss the start of the transit, draw the outline of the solar disk first, and then draw Mercury’s position on it before proceeding with the remainder of the sketch.

Be aware that depending on the type of telescope mount you use, the Sun may rotate in your field of view during the course of the transit. Rotate your sketch as needed so that the orientation of the solar features in the eyepiece view matches those in your drawing. Once the transit is complete, remember to scan your sketch and share it with others for comparison. Good luck!