2017-9-1: Asteroid 3122 Florence

Sky and Telescope has prepared four detailed charts to help observers locate 3122 Florence this week. Two of these show the asteroid’s general motion northward among the constellations. Two more show small areas of sky, plotting all stars brighter than magnitude 9.5, for North American observers on the evenings of August 29-31 and September 1-3. Note that the detailed charts are labeled for Universal Time (GMT), and you’ll have to apply a time-zone correction for your location (for example, 0h UT on August 31st corresponds to 8 pm EDT on August 30th).

During its visit, Florence will be traveling roughly south to north, crossing through the constellations Capricornus, Aquarius, Delphinus, Vulpecula, and Cygnus. An especially good opportunity occurs at about 8 pm Eastern Daylight Time on Saturday evening, September 2nd, when the asteroid crosses the quartet of 4th-magnitude stars that mark the head of Delphinus, the Dolphin. It will be gliding northward by a little less than the full Moon’s diameter each hour, motion that should be obvious by watching the asteroid’s starlike pinpoint through a telescope for just a few minutes.

Florence appears this bright, despite being far away, both because it’s among the largest near-Earth asteroids (2.7 miles across) and it has a fairly bright surface that reflects more than 20% of the sunlight that strikes it. (For comparison, the Moon’s average reflectivity is just 12%.) Although it rotates in just 2.4 hours, this asteroid must be nearly spherical because its brightness varies by no more than about 11% – too small a change to pick up by eye.

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