RHESSI and the Transit of Venus I

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Revision as of 14:41, 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.

Jeremiah Horrocks observes the Sun in 1639.

Iconic portrait of Kepler.

Learning from the Transit

Conclusion