To­tal Lu­nar Eclipse

BEST TIME TO SEE: 21 Jan­uary from 03:34 UT un­til 07:00 UT

Sky at Night Magazine - - THE SKY GUIDE -

The Moon en­ters the Earth’s um­bral shadow in the early hours of 21 Jan­uary pro­duc­ing a to­tal lu­nar eclipse. Given clear skies, the en­tire event will be vis­i­ble from across the UK.

The Earth’s shadow at the dis­tance of the Moon has two parts: a dark in­ner um­bra sur­rounded by a lighter penum­bra, ar­ranged like the in­ner and outer bulls­eye at the cen­tre of a dart board. The Moon’s edge first touches the penum­bral shadow (P1) at 02:37 UT, when it will be at an al­ti­tude of around 20° in the south­west, be­tween Cancer and Gem­ini. How­ever, you prob­a­bly won’t be able to see any signs of this weak shadow at this point.

Look­ing at the Moon’s disc at P1 shows it to be com­pletely full, an event that is rarer than you might re­alise. A ‘nor­mal’ non-eclipse full Moon passes just north or south of the Earth’s shadow. Con­se­quently, a tele­scopic view of a reg­u­lar full Moon still shows a tiny sliver of ter­mi­na­tor shad­ows at the ex­treme north or south of the lu­nar disc.

As the Moon creeps deeper into the penum­bra, the shadow’s in­ten­sity in­creases and you should even­tu­ally be able to de­tect its pres­ence. On page 61 is this month’s Chal­lenge: to see how early you can spot ev­i­dence of the penum­bral shadow.

The edge of the Moon first touches the edge of the darker, cen­tral um­bral shadow (U1) at 03:34 UT, mark­ing the start of the first par­tial phase of the eclipse.

The par­tial eclipse grows in depth un­til the fol­low­ing edge of the Moon reaches the edge of the um­bral shadow (U2) at 04:41 UT. This marks the start of the to­tal eclipse. The Moon con­tin­ues to move into the um­bra, reach­ing the point of great­est eclipse at 05:12 UT. For this par­tic­u­lar eclipse, the cen­tre of the Moon lies half an um­bral shadow ra­dius north of the shadow’s cen­tre at great­est eclipse, so ex­pect the north­ern edge of the Moon to ap­pear brighter than the south­ern edge. Al­though the um­bral shadow rep­re­sents the Sun’s light be­ing com­pletely blocked by the Earth, our at­mos­phere re­fracts sun­light into the shadow. As our at­mos­phere is good at scat­ter­ing blue light, the in­fill light is mostly or­ange and red. Con­se­quently the Moon’s disc will typ­i­cally ap­pear or­ange dur­ing a to­tal lu­nar eclipse. The clar­ity of Earth’s at­mos­phere also has an ef­fect and each eclipse is dif­fer­ent, ap­pear­ing any­thing from a light cop­pery yel­low to a re­ally dark brown that vir­tu­ally dis­ap­pears against the back­ground sky. To­tal­ity ends at 05:43 UT (U3) when the Moon reaches the far side of the um­bral shadow. A sec­ond, de­creas­ing par­tial eclipse fol­lows with the Moon leav­ing the um­bral shadow com­pletely at 06:51 UT (U4) as the dawn sky starts to brighten. The de­par­ture from the penum­bra will be com­plete at 07:48 UT (P4), with the Moon hang­ing low above the west-north­west hori­zon in a twi­light sky.

As the end of to­tal­ity ap­proaches at around 05:43 UT (U3) the western edge of the Moon will ap­pear to brighten

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