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--JeffGBot (talk) 18:56, 1 June 2011 (UTC)[reply]

The last diameter

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I got confused by this too but then realized that ~110 km is the width along the Earth's surface. Per [1], [2] and basic geometry, the "umbra" is 1.371.2km smaller than the asteroid, assuming average asteroid diameter. Maybe there should be a clarification. Note: Occult Watcher software says 107km at NYC. Sagittarian Milky Way (talk) 23:37, 19 March 2014 (UTC)[reply]

Also, Regulus says 79 light years is closer, not 78. Sagittarian Milky Way (talk) 23:43, 19 March 2014 (UTC)[reply]
It is good to realize and note that, but this does not resolve the discrepancy between ~73 km and ~107 km. --JorisvS (talk) 07:59, 20 March 2014 (UTC)[reply]
occultation path width clearly states 107km. -- Kheider (talk) 08:45, 20 March 2014 (UTC)[reply]
Sure, but that does nothing to resolve the discrepancy. Also, how is "path width" distinct from "size of the shadow"? --JorisvS (talk) 08:48, 20 March 2014 (UTC)[reply]
I assume it is because asteroids are 3 dimensional objects with three axes and we do not know if the longest or shortest axis blocked the light from Regulus. I assume it is more of an upper bound on the expected result. -- Kheider (talk) 08:54, 20 March 2014 (UTC)[reply]
Can we find anything that could support that? --JorisvS (talk) 09:05, 20 March 2014 (UTC)[reply]
Alas, the occultation was clouded out for me and I went to sleep. Anyway, the shadow of a ball upon a flat surface is usually longer than the ball. Just try it with a beach ball on the parking lot. Except when the ball and Sun are directly overhead, of course. The event in question was not overhead; it was about as close to horizon as to zenith, so the shadow was appreciably longer than the roughly ball-shaped asteroid. And it was stretched in a direction perpendicular to the path of the shadow on the ground, thereby producing an eclipse along a wider swath than the width of the asteroid. No, it's not an umbra / penumbra distinction. Distant stars subtend too small an arc to make that big a difference. Jim.henderson (talk) 12:00, 20 March 2014 (UTC)[reply]
Yes there is an umbra / penumbra distinction. The umbra is the cone that touches the star's edge and asteroid edge. (assuming the asteroid's average width because we don't know it's shape or orientation) Divide the amount this cone shrinks by Regulus's distance in kilometers. This is how much the umbra shrinks for each kilometer away from Regulus. Multiply that by Erigone's distance to get the total amount of shrinkage from Erigone's size. Regulus is so far away that the ambiguities in this description don't matter. Therefore unless we get more data the asteroid is 73km, the umbra 71km or 72km, and the penumbra 74. The circa 107km path width is similarly ambiguous. I made a mistake by using the star's (huge difference!) major axis diameter instead of average, as pole position angle (at the bottom) and Erigone path PA shows. Sagittarian Milky Way (talk) 23:10, 20 March 2014 (UTC)[reply]
It was hard for me to believe that such a far away light source could have a penumbra when an asteroid passes in front of it, but your math is right, and instructive! :)  — TimL • talk 05:29, 22 March 2014 (UTC)[reply]
Can we cite the apparent location of Regulus at the time? --JorisvS (talk) 12:15, 20 March 2014 (UTC)[reply]
Nope. From North River Pier 84 I saw Regulus about a week earlier, a few hours earlier in the evening. It was near meridian, over Chelsea, about 50 degrees high. I merely assume, at occultation time, it was several degrees lower. This makes for a shadow projection exaggeration of some 30 km, or much bigger than the twilight zone width (thanks, @Sagittarian Milky Way). 20:16, 23 March 2014 (UTC)[reply]
"The occulting asteroid diameter is 72 km: this is the width of the totality band on the Earth in the case of zenital occultation, the projection from the altitude of Regulus gives an ellipse of ∼ 144 × 72 km."[3] Praemonitus (talk) 16:21, 13 December 2021 (UTC)[reply]
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