RHESSI and the Transit of Venus I

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Revision as of 15:16, 1 June 2012

Introduction

Transits of Venus across the solar disk happen only rarely, but in pairs. In this Nugget we describe how RHESSI benefits from them; the first of our pair was in 2004, and the next (and the last one for more than a century) occurs June 5-6, 2012. Note that this has nothing directly to do with RHESSI's main goal of X-ray and γ-ray imaging and spectroscopy, but instead with its optical aspect system - a set of three simple telescopes with 4-cm apertures.

Historically, we can point to the 1639 observations by Jeremiah Horrocks in the village of [2], Lancashire. Figure 1 shows this worthy gazing at a projected image of the Sun, and one can see the shadow of Venus creeping across it. The whole process takes several hours. The shadow is comparable in size to a sunspot, but - depending on the telescope and its environment - much darker. Horrocks apparently improved on Kepler's novel adoption of ellipses for planetary motions; with no formal training in mathematics or physics (but how much was there in 1639?) he improved the orbital elements, predicted the occultation, and then observed it with his own home-made telescope. Not much is known about any of this, but from this observation he was able for the first time to determine the vastness of the scale of the solar system - the distance to the Sun could not be known without this crucial observation.

Left, the iconic portrait of Johannes Kepler; right, a not-so-well-known portrait of Jeremiah Horrocks as he makes the first scientific use of a transit of Venus across the solar disk, in 1639.

There is a great deal of history involved in transit observations, but 1639 was a remarkable beginning.

Learning from the Transit

No longer do we need such astronomical observations, in fact our vehicles have actually gone out there and visited Venus (1962), and have gotten very near the Sun (1976) as well. But the two transits of the current cycle (2004 and 2012) are very significant as a convenient tool for calibrating telescopes, especially those in space (such as RHESSI). The main areas of calibration deal with learning about the optics of these telescopes, referring to the exquisitely precise knowledge we now have regarding celestial mechanics. For example, we now know the value of the one AU to be 149,597,870.700 km, as compared with Horrocks' first determination of some 95,000,000 km. It also took him about 20 years to get his result published, something that wouldn't readily be tolerated in the modern era.

Figure 2 shows a RHESSI aspect-system (SAS) observation from the 2004 transit. The SAS optics are read out by a 2048-pixel linear CCD, and so this image just shows a thin strip that intersects the solar limb in two places, and crosses over the shadow of Venus in between.

Example of a RHESSI/SAS scan (parts of a 1D slice across the phenomenon) from the 2004 transit.

Conclusion