The Planetary Transit Special Award

Planetary Transit Special Award Coordinator:

Aaron B. Clevenson
19411 Cluster Oaks Drive
Humble, TX 77346-2918
(281) 852-4667

These Planetary Transit Special Award opportunities have passed and no new submissions are being accepted:

  1. Venus Transit 2004
  2. Venus Transit 2012

The Al Council has adopted the next award in this series:

  1. Mercury Transit 2016



Welcome to the Astronomical League's Planetary Transit Special Award site.  The purpose of this award is to provide an opportunity to relive the excitement of being on a planetary transit expedition and to derive a value for the Astronomical Unit, and to recognize those who participate.  This award is being presented in collaboration with NASA.

Planetary Transits are rare events.  They occur when a planet (Mercury or Venus) passes in front of the sun as seen from Earth.  This requires the the inner planet be at inferior conjunction along the line formed by the intersection of the Earth’s orbital plane and the planet’s orbital plane (the line of nodes).  Only when these two conditions are met will a transit occur.

Although materials for this program are still being developed, some of the details are known.  We will update this site as we get closer to the event and have more details finalized.


*** Safety Warning ***

Observing the Sun can be dangerous if not done correctly.  Never look at the sun directly.  Even when it is low on the horizon and doesn’t appear to be dangerously bright, it is hazardous.  When the sunlight passes through a lot of atmosphere, as it does when it is near the horizon, the blue light is scattered, so less overall light reaches you.  But the red and infrared light passes through relatively undimmed and can harm your eyes.  So do NOT ever look at the sun.  Use one of the safe techniques mentioned below.


Information on the Next Planetary Transit: Mercury 2016

The next planet to transit the sun is the planet Mercury on May 9, 2016.  Details on transit are available from NASA on this site: http://

Note that for those of us in the eastern United States, the entire transit will be visible. For those in the western United States, the transit will be already in progress at sunrise, so we will not be able to observe the entire event. Timings for the contacts are:

               Event                                   UT

               Contact I                               11:14, May 9, 2016

               Contact II                              11:17, May 9, 2015

               Maximum Transit                  14:58, May 9, 2015

               Contact III                             18:39, May 9, 2015

               Contact IV                             18:42, May 9, 2015

Definition of contact points:

  • Contact I:  when the leading edge of the planet first touches the edge of the Sun.
  • Contact II:  when the planet is initially completely on the disk of the Sun.
  • Maximum Transit:  when the planet is at maximum transit (the middle).
  • Contact III:  when the leading edge of the planet first reaches the far edge of the Sun.
  • Contact IV:  when the planet is initially completely off the disk of the Sun.

Mercury is small and so far from Earth and  its silhouette is visible only with magnification, but be sure to use proper filters to protect your eyes.

(The next transit after this one is the planet Mercury on November 11, 2019.  Mark your calendar.)

How to Safely Observe the Transit:

Observing the Sun can be very dangerous.  Be very careful.  Never look at the Sun without proper filters.  A safe filter must filter 99.999% of the sunlight at visible as well as invisible wavelengths (infrared and ultraviolet).  Damage can be immediate and permanent!

Option 1: Pinhole Projection.  This is a great, low-tech way to watch the event.  It lets multiple people see it at the same time.  Do NOT look through the hole at the Sun.  Put a small pinhole in the center of a card.  Let the light coming through the hole hit a large white card or piece of paper about 3 feet away.

Option 2: Eyepiece Projection.  This technique will work for a small telescope or a pair of binoculars mounted on a tripod.  Do NOT look through the binoculars or the telescope when it is pointed near the sun!  Do NOT do this with a large telescope, the optics may overheat and be permanently damaged.  If you are using binoculars, cover one of the objective lenses.  Point the equipment at the Sun.  Then hold up a white card or piece of paper so that the light coming out of the eyepiece is projected onto the paper.

Option 3: Solar Sunglasses.  Because of Mercury's tiny angular diameter its disk will NOT be visible through Solar Sunglasses.  These sunglasses are easily available on the internet and will be available in local stores as the time draws near.  Once you have them, check to be sure that there is no damage.  Put them on, and look at a bright incandescent light bulb. You should be able to see the glowing filament. Make sure that there are no cracks, creases, or pinholes. Even a small pinhole will let in a potentially dangerous amount of light from the Sun.  It you see any damage at all, do NOT use them. Also note that there are NO sunglasses that are safe for looking at the sun.

Option 4: Binoculars or a Telescope with a Solar Filter. They should have a filter at the front end of them. Do NOT use solar filters that attach at the eyepiece end. Make sure that there is no damage on the filter before you use it.  Be sure to cover the finder scope.

Option 5: A Solar Telescope. These are telescopes specifically designed for observing the sun usually at specific wavelengths  (e.g. H-alpha – 656.3 nm, Calcium K – 393.4nm). I believe that the view in a standard telescope with a standard solar filter will be more pleasing.

Option 6: Shade #14 Welder’s Glass.  Because of Mercury's tiny angular diameter its disk will NOT be visible through Welder's Glass.  It is critical that you use shade #14 or higher. The higher number the more filtering.  If you are unable to get the shade #14 welder’s glass, then we recommend that you use one of the projection methods (options 1 or 2 above). If you combine more than one piece, your total must be higher than 14.  Two pieces should total 15. Three should total 16.  If you use more than one piece, tape them together so that you will always be sure to be looking through all of the filtering. A higher number is always better.

Historical Information:

Pierre Gassendi, a French astronomer, is credited as being the first person to witness a transit of Mercury in 1631.  He tried to observe the transit of Venus a month later, but it was not visible in Europe.  Jeremiah Horrocks and William Crabtree, British astronomers, observed the next transit of Venus on December 4, 1639.

In 1677, Edmond Halley observed a transit of Mercury from Saint Helena's Island. He realized that the careful timing of transits could be used to determine the distance of Earth from the Sun. His technique relied on observations made from distant locations through measuring parallax.  (Parallax is the angular shift in the location of a nearer object against a distant background due to a change in observing location.)  Taking this measurement would enable astronomers to calculate the value of the Astronomical Unit (Earth’s distance from the Sun) and in turn calculate the distances to all of the planets. Venus transits provide a more accurate parallax measurement than Mercury transits because Venus is closer to the Earth resulting in a larger parallax.

Expeditions to observe the transits in 1761 and 1769 did not yield useful results.  Another set of observations of Venus were done in 1874 and 1882. Once again, the results were not of high quality.  One problem plaguing these expeditions is an effect called the "black drop".  The edge of the planet's disk appears to deform and cling to the limb of the Sun.  The "black drop" is the result of seeing problems due to the Earth's turbulent atmosphere.

Rules and Regulations:

There will be a downloadable certificate available for anyone hosting a transit viewing.  The host can then provide these certificates to those who participate as a memento of their experience and to commemorate this very rare experience.  It will be jointly from the Astronomical League and NASA.

There will also be a NASA Observing Challenge special observing award for those who follow the rules on the NASA website:  Take an image, submit it to the NASA Flicker website, conduct an outreach event, and fill in the form on the NASA website.  There is a certificate for this challenge from the Astronomical League and NASA.  You do not have to be a member of the Astronomical League to earn this certificate.  Deadline for submissions is July 9, 2016.  No late submissions will be accepted.

The Astronomical League will also have a Special Observing Award to commemorate the event. Measurements and calculations will be required to earn this award.  (details are forthcoming)  The special award will include a certificate and a pin.  Deadline for submissions is July 9, 2016.  No late submissions will be accepted. The pins will be ordered after this deadline.  This is only available to members of the Astronomical League.

The certificate for anyone viewing the Mercury Transit is

not yet available.

Planetary Transit Special Award Coordinator:

Aaron B. Clevenson
19411 Cluster Oaks Drive
Humble, TX 77346-2918
(281) 852-4667