astroengine writes "On Tuesday, Venus will race across the sun — the last Venus transit until 2117 — so the world's astronomers are primed to view this enigmatic event. But it's not just for the historical significance of the celestial phenomenon; real science will also be done. Just before the transit begins, as Venus sinks into the Sun's limb (an event known as "ingress"), the atmosphere of the planet is expected to display a crescent glow known as an "aureole." For as long as transits have been recorded, the Venus aureole has been observed. They are caused by sunlight being refracted through the atmosphere. Interestingly, the aureole is not uniform — often a bright spot appears around the planet's poles. This enhancement in brightness is caused by a variation in atmospheric temperature in the polar mesosphere. When detected, astronomers have the great opportunity to gauge the temperature in the Venusian upper atmosphere, comparing their results with atmospheric models and in-situ measurements made by the ESA Venus Express spacecraft currently in orbit around the planet."