May 30, 2020

3.8.9: Jupiter Occults a Bright Star.

Lots has been afoot in the Jovian system as of late. As you train that 10” Dobsonian on the ever evolving black spot gracing Jupiter’s cloud tops, I turn your attention to another unique event about to occur tonight; the occultation of a bright star by the large gas giant. The star is 45 Capricorni, which is currently crossing our line of sight with Jupiter. At about sixth magnitude, it will masquerade as a Galilean satellite over the coming days.The actual occultation begins at 23:00 Universal Time (UT) on August 3rd and lasts until 1:00 UT on the 4th. Europe, Africa, the Canadian Maritimes and extreme northern New England will be well placed to see this rare occultation; the remainder of the Americas will see 45 Cap rise with Jupiter at about 9 P.M. local. An occultation of a bright star by a planet is rare because planets are intrinsically small targets in terms of visual diameter, and stars that they can occult are constrained to those along the path of the ecliptic. Speaking of which, the four large moons of Jupiter are also currently under going a fascinating series of mutual eclipses as we transit their respective orbital planes; check out the link above for more info, and watch the occultation of 45 Cap if you get a chance. Some things to watch out for; does the star “wink in, wink out” in a step wise fashion, or fade gradually in and out? You could be seeing evidence of Jupiter’s atmosphere refracting the starlight; or perhaps this is glimpse 45 Cap’s binary companion! Also known as HIP 107302, this star is also listed as a close spectroscopic double. This will also be the brightest star that Jupiter has occulted since 1952.

This weeks astro-term is shadow transit . One of the very cool things about the Galilean moons is that their orbit is often aligned with our line of sight… this means that shadow transits across the disk of Jupiter are a common event. Each shadow cast presents a different look; close in Io tends to be large and diffuse, while far out Callisto is tiny and sharp. Rarer still are double or triple transits. I bring this up because on the night of August 5th, a double overlap transit of Ganymede and Europa occurs, again favoring European latitudes. And don’t forget to peek at nearby Neptune, just degrees north of Jupiter…did you know that in January 1613, Jupiter actually occulted Neptune, and Galileo even sketched the motion of the planet through the Jovian system? Perhaps the reason he failed to recognize the planet was because it never occurred to astronomers at the time that there should be undiscovered planets!


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