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 1883; 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 Jer=F4me 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 transit
of Mercury, visit:
http://gong.nso.edu/mercury_transit03.

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.

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