Preparing for our second transit of Mercury

Science Nugget: November 12, 1999


Only infrequently does the planet Mercury pass across the face of the Sun, something like once per decade (for a richly detailed discussion please see Fred Espenak's transit99 pages; si usted habla español vea, por favor aqui). We will see the first transit of Mercury in 6 years on November 15, 1999. It may not be easy to observe from the ground, but from space, where Yohkoh, SOHO, and TRACE fly, it is a different matter. Yohkoh in fact observed the last previous transit of Mercury in X-rays, almost an observational first except that the NASA Skylab astronauts observed one in the 1970s! But we got pretty pictures.

For Yohkoh the payoff of this delicate and rare observation was that SXT scientists Wuelser and Nishio were able to calibrate the Yohkoh roll angle against the known position of Mercury in the sky -- its orbit has been known exquisitely well, by astronomical standards, since the 19th century. Transits have been a key to that knowledge, and it's well-known that the mutiny on the Bounty had something to do with a transit of Venus!

Predictions for Yohkoh, TRACE, and SOHO

The visibility of a transit depends upon where you observe from -- the planet has to be between you and the Sun. Transits from satellites are especially tricky, since they move so rapidly; we are fortunate to have Yohkoh and TRACE predictions, made by M. Soma of the National Astronomical Observatory of Japan. We present these here (compare and contrast!).

First Yohkoh:

The image on the right is larger-scale but less interesting from the point of view of solar pulchritude. Here the dots show where the satellite is in sunshine, and the pluses where it's also in the Van Allen radiation belts in the Earth's magnetosphere.

Now, how will it look from the TRACE satellite? This spacecraft is in a polar orbit, so it has more NS parallax (orbital variation of the apparent position of Mercury relative to the Sun) than does Yohkoh.

On the left, a similar plot to the first Yohkoh plot; on the right, one with a magnified NS axis. The Sun looks oval but now one can clearly see the wobble due to the orbital motion of TRACE in its polar orbit, which has a much larger north-south excursion than the low-inclination Yohkoh orbit.

Now, how will it look from SOHO?

Be prepared to wait for the full SOHO .jpg file, it is 443 kB. But it's nice (image courtesy of F. Auchère). Here one sees that Mercury does not cross the solar disk. This is because SOHO is in a "halo orbit", looping the loop in a so-called Lissajous figure around the L1 Lagrangian point (a point some 1% of the distance between Earth and Sun where a stable orbit is possible). We can guess, after careful thought, that SOHO is S of the ecliptic plane, and the ephemeris for the satellite motion puts it 130,000 km south according to SOHO guru Joe Gurman. This would make a parallax of a few arc minutes, enough to do the trick. The figure below shows the complicated SOHO orbit:

Sad and happy news

First the sad news. Yohkoh, by an unfortunate coincidence, will have no "roll reference" on Nov. 15. That's because the sensor that uses the bright star Canopus to figure out the orientation of the satellite along the Sun-Earth line is temporarily blocked from view. This happens regularly, and we just got unlucky, so we were not able to use the Mercury images to calibrate this sensor, as we did in 1993. So it just becomes an astronomical curiousity for us this time.

The happy news -- we'll get another chance in a few years:

  • Next Mercury transit: May 7, 2003
  • Next Venus transit: June 8, 2004


    November 12, 1999
    Hugh Hudson (
    Brian Handy (