Comets and As­ter­oids

Fol­low 15 Euno­mia as its skims along the hori­zon to meet Cen­tau­rus

Sky at Night Magazine - - CONTENTS -

As­ter­oid 15 Euno­mia was dis­cov­ered by An­ni­bale de Gas­paris in 1851 and is the largest known S-type, or sil­i­ca­ceous, as­ter­oid – these are char­ac­ter­is­ti­cally metal­lic, re­flec­tive and bright. In May, Euno­mia will hover around mag. +10, mak­ing it an easy tar­get for a small tele­scope or large binoc­u­lars. It reaches op­po­si­tion on 8 May when it will ap­pear as a mag. +9.8 dot in the north of Cen­tau­rus.

Cen­tau­rus isn’t nor­mally as­so­ci­ated with north­ern hemi­sphere view­ing but it’s a glo­ri­ous sight to be­hold from more southerly climes. How­ever, the ex­treme north­ern part of the con­stel­la­tion just man­ages to rise above the UK’s south­ern hori­zon. In order to see it, you’ll need a good, flat hori­zon in this direc­tion. Aim to view from 01:00 BST (00:00 UT) at the start of the month. A bright, al­most full Moon will spoil the show dur­ing early May and a se­vere lack of night will make things harder at the end of the month. Iron­i­cally this is when the as­ter­oid will be climb­ing higher in the sky. The best time to catch Euno­mia will be mid-month from around 00:00 BST (23:00 UT).

At 330kmx245k­mx205km Euno­mia is be­tween the eighth and 12th largest as­ter­oid in the main-belt be­tween Mars and Jupiter. It’s not pos­si­ble to rank ac­cu­rately as there’s un­cer­tainty about some di­am­e­ters. Its or­bit swings from 2.149 AU out as far as 3.138 AU from the Sun. The or­bit is chaotic and will change ran­domly over time be­cause of the grav­i­ta­tional in­flu­ence of the plan­ets. It cur­rently takes 4.3 years to com­plete one or­bit and ro­tates once on its axis ev­ery 6.1 hours.

It can ap­pear as bright as mag. +7.9 dur­ing favourable op­po­si­tions, with an an­gu­lar di­am­e­ter of 0.29 arc­sec­onds. Its mass is es­ti­mated at slightly in ex­cess of 1% of the en­tire as­ter­oid belt at 3.12x1019kg.

You need a flat, clear hori­zon for the best views of 15 Euno­mia

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