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ON THE NIGHT of April 28-29, the asteroid 20 Massalia will hide a 10th-magnitude star in the Twins for observers in the southeastern United States. This Classically inspired location seems appropriate for this event: Massalia's designation comes from the Greek name for Marseille, one of the cities from which the asteroid was discovered. (French astronomer Jean Chacornac and the Italian astronomer Annibale de Gasparis both spotted the asteroid on September 19, 1852.) At 160 km x 145 km, Massalia is one of the largest members of its eponymous family, a group of stony, silicaceous asteroids with similar orbital elements and compositions. In this case, 20 Massalia is the indisputable head of the clan. The smaller members of the family are actually fragments of Massalia, ejected by a cratering event large enough to do some damage, but not so great as to thoroughly destroy the parent.

As Massalia passes in front of the magnitude-10.2 star TYC 1357-02401, their combined light will drop to 10.9 (Massalia's magnitude) for a maximum of 4.7 seconds. This dip in brightness will occur within a couple of minutes of 1h 18m UT on April 29th (9:18 p.m. EDT April 28th), just before the end of astronomical twilight for Florida.

About a week before the event, more precise predictions and a path map will be available from Steve Preston's minor-planet occultation website ( For advice on timing occultations and reporting observations, see Occultation enthusiasts can join an online discussion group at

Caption: Every 2.7 days, Algol (Beta Persel) dips from its usual magnitude 2.1 to 3.4 and back. Use this chart to carefully estimate its brightness with respect to the convenient comparison stars of magnitude 2.1 (Gamma Andromedae) and 3.4 (Alpha Triangull).

Minima of Algol

Mar.    UT     Apr.    UT

2      20:43    3     9:45
5      17:32    6     6:34
8      14:22    9     3:24
11     11:11    12    0:13
14     8:00     14    21:02
17     4:50     17    17:51
20     1:39     20    14:40
22     22:28    23    11:29
25     19:17    26    8:18
28     16:07    29    5:07
31     12:56

These geocentric predictions are from the
heliocentric elements Min. = JD 2445641.554 +
2.867324E, where E is any interger. For a naked-eye
comparison-star chart and more information about
Algol and its history, see
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Title Annotation:APRIL 2018 OBSERVING: Celestial Calendar; asteroid 20 Massalia
Author:Johnson-Roehr, S.N.
Publication:Sky & Telescope
Date:Mar 18, 2018
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