The Sky Guide Chal­lenge

Cap­tur­ing the shadow of Venus

Sky at Night Magazine - - CONTENTS -

From Earth, Venus ap­pears in­tensely bright shin­ing away in the evening or morn­ing twi­light sky. If you catch it right, it can also be seen against a dark sky, but the con­di­tions for this can be tricky to achieve. If the sky is dark enough, the in­tense light from Venus can do some­thing ob­vi­ous but still rather amaz­ing – it can cast a shadow. This month’s chal­lenge is to pho­to­graph the shadow cast by Venus.

Venus’s light may be in­tense, but there’s not much of it. Con­se­quently, any other light will swamp it com­pletely. One way to over­come this is to cre­ate an en­vi­ron­ment where the light from Venus is dom­i­nant. One way of do­ing this is to make use of a room or an area in­side a house where there’s a win­dow fac­ing in the cor­rect di­rec­tion and which will let light from Venus en­ter and fall on a build­ing wall. If this isn’t fea­si­ble, an­other tech­nique is to cre­ate an ar­ti­fi­cial ‘room’ – say with a card­board box – that can be po­si­tioned in such a way that a sin­gle open­ing points at Venus. Both meth­ods re­quire a tar­get that will be used to make the shadow. This can be any­thing you like, such as a shape cut from card at­tached to a win­dow or, in the case of the card­board box, sus­pended in the open­ing. The shape we opted for was the sym­bol for Venus.

Fi­nally, the ‘screen’ wall will work bet­ter if it is smooth and white. A piece of white

pa­per or card is ideal for this. This can be tem­po­rar­ily at­tached to a wall or stuck to the sur­face of the box op­po­site the open­ing.

To show the shadow at its best, a good tech­nique is to cre­ate an an­i­ma­tion of the shadow mov­ing as Venus ap­pears to set. A DSLR cam­era on a tri­pod is good for this, but any cam­era that al­lows a high ISO and multi-sec­ond ex­po­sure should work too.

Set ev­ery­thing up on a clear evening. Imag­ine where the light from Venus will be fall­ing and cast­ing the shadow – ba­si­cally a straight line from Venus through the tar­get and onto the screen. Set a high ISO, fully open the lens and aim to use an ex­po­sure of 10–30 sec­onds. Fo­cus on the screen care­fully. You may want to tem­po­rar­ily il­lu­mi­nate it to do this ac­cu­rately.

Take the first im­age. Wait one minute and then re­peat. Flick­ing be­tween the two im­ages should be enough to re­veal the weak shadow. The shadow is in­cred­i­bly sharp be­cause Venus is ef­fec­tively a point source of light. How­ever, the longer your ex­po­sure, the fuzzier the shadow will be­come be­cause its edges are es­sen­tially mov­ing.

Shadow An en­hanced im­age of the shadow cast by Venus onto the in­ter­nal wall of a house

The tar­get for our shadow im­age was the sym­bol for Venus cut out of card. Dis­tant trees also cre­ated a mot­tled ef­fect in the fi­nal an­i­ma­tion

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