A Press Release Regarding
The Transit of Mercury from NSO

National Solar Observatory Press Releases
Mercury Transit 2003
IMMEDIATE RELEASE: May 5, 2003

Contact: Dave Dooling
NationalSolar Observatory
P.O. Box 62
Sunspot, NM 88349
505-437-2294
dooling@nso.edu

Modern solar telescope network's view of Mercury passage will help students use web to recall historical era.

A global network of telescopes designed to watch the Sun's atmosphere pulsate will be pressed into service on May 7 to help students recreate early measurements of our solar system.

The telescopes will record the transit of Mercury as it crosses in front of the Sun. Transits once were the most valued of astronomical events, a rare chance for astronomers to size up the solar system. Today it is an opportunity to involve science teachers and students in studying both the Sun and mathematics.

The observations will be made by the National Solar Observatory's Global Oscillation Network Group (GONG) telescopes located in Australia, India, and the Canary Islands.

"We were approached by a French colleague, Professor Michele Gerbaldi of the Institut d'Astrophysique in Paris, Maitre de Conferences at the University of Paris-Sud, Orsay," explained Dr. Cliff Toner, the GONG scientist who is spearheading the transit observations. "She wanted to redo the work of a French expedition in the late 18th century to measure the scale of the solar system with modern data of the transit of Venus, observable next year, the one of Mercury this year being used as preliminary just as it has been the case in the 18th century. It has tremendous historical value, and it is awesome what those people were able to accomplish."

Transits occur when Mercury or Venus passes between Earth and Sun. The timing is complex and depends on the relative motions of Earth and the other planet. Mercury transits in May at intervals of 13 and 33 years, and in November at intervals of 7, 13 and 33 years. GONG observed the last transit of Mercury on Nov. 15, 1999. Venus is less frequent, only six times in the last four centuries. The last was 1882; the next will be very soon, on June 8, 2004.

In the 17th century, pioneering work by Jeremiah Horrocks (an English astronomer) and James Gregory (a Scottish astronomer) demonstrated that the transits could be used to determine the Earth-Sun distance. In 1716 Sir Edmund Halley published "A new Method of determining the Parallax of the Sun, or his Distance from the Earth" by using many observational stations spread over the world. But Halley's own expedition to the South Atlantic in 1677 to observe the transit of Mercury came to naught when bad weather in England deprived him of the other half of the observations.

Several nations mounted expeditions in 1761 and 1769 to observe the transits of Venus and produced measures of the Earth-Sun distance. Using those data, Joseph Jerôme Lalande of France in 1771 calculated the Earth-Sun distance at 153 million km (95 million miles), just 3.4 million km (2 million miles) off the correct number, 149,597,871 km (92,750,680 miles). Today, radar ranging to the planets and tracking of deep space probes have relegated transits to reminders of the pioneering days of astronomy.

But GONG's constant watch on the Sun means that we don't have to mount a special expedition. Three GONG stations will see the 5-hour, 19-minute transit. It starts at 05:12:56 Universal Time (12:13 a.m. EDT) when Mercury's limb appears to touch the Sun's limb, and ends at 10:31:46 UT (5:13 a.m. EDT) when Mercury clears the Sun. Teide will see the first 3-1/2 hours (from sunrise), Udaipur, India will see the entire transit, and Learmonth, Western Australia, will see the last 3-1/2 hours (to sunset). Learmonth and Teide will overlap each other by almost 2 hours. So while the transit will occur entirely at night for half the world, people anywhere should be able to see it.

CAUTION: It is exceptionally dangerous to view the Sun without the right equipment. Blindness or painful, permanent eye damage will result.

"While we don't expect the size of the Universe to change as a result of these measurements" said Dr. John Leibacher, the GONG program director in Tucson, AZ, "it is an exciting spectacle to watch, and it is of important practical use to us in establishing the precise orientation of the images taken with different GONG telescopes around the world."

GONG was designed to measure the pulsations of the visible surface of the Sun's atmosphere as it rings like a bell with millions of different harmonic notes. These vibrations are our only way of probing the Sun's interior, just as earthquakes probe Earth's interior. Six identical GONG stations around the globe monitor the Sun full time: Big Bear Solar Observatory, Big Bear Lake, Calif.; Learmonth Solar Observatory, Australia; Udaipur Solar Observatory, India; Observatorio del Teide, Canary Islands; Cerro Tololo Interamerican Observatory, Chile; and Mauna Loa Observatory, Hawaii. Thus, the Sun never sets on GONG, making it uniquely suited to catching a transit whenever it may occur.

Leibacher explained that software has been developed for the GONG network computers to extract one image every 15 minutes from each site as Mercury crosses the Sun and post the image in near real-time on the GONG web site. Only 25 or so images will be posted for this quick-look, stop-motion movie of the transit. Toner cautioned that the real-time connection with Udaipur is new and may experience some interruptions. The connections with Learmonth and Tiede, though are working well and the overlap between the two will ensure continuous coverage.

Over the next two months, as data tapes arrive from the GONG sites, the GONG team will prepare an education CD-ROM with raw transit images taken every minute for a total of more than 300 images.

"We'll provide the raw data from the white-light images, so the students can learn what is the triangulation method and how to measure the Earth-Sun distance from planetary transits and be prepared for the transit of Venus, next year which is the one allowing a measure of the astronomical unit," Toner explained. Reproducing the timing aspect of the early experiments may not be possible because each image will have an integration time of one minute, too long for making precise contact measurements. The CD-ROM will include instructions on how to use the images and data.

"This is the first time that we have tried something like this, so everyone here is pretty excited," said Leibacher, "and it's just a warm-up for the transit of Venus next year."

GONG is operated by the National Solar Observatory under contract to the National Science Foundation.

For additional information on GONG and the 2003 transit of Mercury, visit: http://gong.nso.edu/mercury_transit03. For a larger image of the 1999 transit, visit the NOAO Image Gallery. [

Editor's note: Historical and technical information on transits is drawn from the transit pages maintained by Fred Espanak of NASA's Goddard Space Flight Center: http://sunearth.gsfc.nasa.gov/eclipse/OH/transit03.html.

Halley's paper on determining the Earth-Sun distance is republished at http://sunearth.gsfc.nasa.gov/eclipse/transit/HalleyParallax.html.  

Dave Dooling and Ruth A. Kneale
05/06/2003 15:12:03

The National Solar Observatory is sponsored and supported by the National Science Foundation.