Zero Shadow Day

It is said that the your shadow never leaves you but what if it does so – even just for a while?

Well this is exactly what can be experienced by those living between the tropic of Capricorn and tropic of Cancer. That is between the latitudes (-)23.5 deg. south (+) 23.5 deg. north. Not just once but twice every year, between about 22nd of Dec to 21st of June and then between 21st of June to 22nd of December.

On one of the days – depending upon one’s latitude  the sun is right overhead and for a short time the shadow of a vertical object is right below the base – as if it has disappeared.

We  called it Zero Shadow Day.

It is very exciting to watch ones shadow or that of any vertical object to become smaller and smaller and then to disappear for a few tens of seconds and the to reappear again on the other side.

What started about 2005 or so as a part of summer programme for the school students at Inter-University Centre for Astronomy and Astrophysics, Pune, India, became quite very popular with the press and general public.

On the specific day all one need to do is to keep a vertical object in the sun and watch it’s shadow becoming smaller and smaller and then falling right below it, as if it has disappeared.

The phenomenon itself is quite enjoyable and it carries a high value of edutainment. We used this event to talk about the curvature of the earth and how Eratosthenes found the circumference of the Earth.

Here are some examples. These pictures are taken on the Zero Shadow Day at time when the Sun is exactly over head or at Zenith.

      
A stainless steel box that has a bigger cap A flash light – its strap will of course cast its shadow A marker pen that was made to stand on its narrow part

The effect is quite dramatic with a vertical hollow pipe –

——–

The girl is holding a plane sheet of glass right over the pipe
The Sun subtends an angle of half a degree at the Earth and thus takes a minute to travel half its angular diameter across the sky. The phenomenon is observable for a minute or so.

At the time when the sun is right over head she see the bottom of the pipe. The picture above was taken soon after the picture to the left was clicked.

——–

The experiment is even more dramatic and enjoyable if we keep a hallow tube (such as the one for shuttlecocks) on a sheet of glass which is made perfectly horizontal.We have used here four paper cups to rest the glass sheet, which has been cleaned thoroughly with soap and tap water. The Tube should be able to stand on its own and should not topple over by light wind. It the tube is too long you may cut it. It is important that the end of the tube that would rest on the glass should be perfectly perpendicular to its axis.  It does not matter if the other end is a bit uneven or jagged.Note that the glass sheet should be perfectly horizontal.One could use a spirit level and using wooden or paper wedges to make the glass sheet horizontal.

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If you do not have a spirit level you can use a glass tumbler.As shown here, use a pointed tipped over head transparency marker. Rest the tip of the pen on the tumbler at a certain height. You can use a stack of books to get the proper hight. Draw a line on the wall of the tumbler by simply rotating its base. This line would be parallel to tumbler’s base.
Place the tumbler on the sheet of glass and fill  it with water to the levelling line.If the base is not perfectly horizontal then the water level will be above or below at some point on this line. Using paper or wooden wedges make the glass sheet can be made perfectly horizontal.Place the cardboard tube on the glass sheet without removing the tumbler.You are ready for the show

The sequence of images below show the progress of the shadow as the sun passes over head. You can see that almost for two minutes the shadow of the hollow tube and the glass tumbler are perfectly circular.

12:10     12:15

12:20     12:25

12:26     12:27

12:28     12:29

12:30     12:40

12:45      12:50

If you are going to take a sequence of pictures of this event then it is good to remember that you will be standing in the Sun that will be right over head. Have a big umbrella and a hat over your head – keep the camera on a tripod and bottle full of cold water – it will become warm in no time.

Check out which places in India would experience the Zero Shadow Day moment and when – Click here.

What is happening: Click here for explanation of this phenomenon.

Who does not experience ZSD: for those north of tropic of Cancer (between +23.5 and +90 degree latitude) and south of tropic of Capricorn (between -23.5 and -90 degree latitude) will never experience the Sun right overhead.

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About skytonight

I am the present Director of Nehru Planetarium of Nehru Centre Mumbai, India I like to talk about astronomy and sky observations to general public.
This entry was posted in Interesting events and tagged , , , . Bookmark the permalink.

4 Responses to Zero Shadow Day

  1. Pingback: ZSD over Indian Cities « Eyes have it – list of some beautiful celestial events

  2. Kshitija Deshpande says:

    Really wonderful website! We appreciate how you bring these ideas forth with simple nice experiments…

  3. Arun Venkataswamy says:

    Nice practical experiments with everyday objects. Nice post. Hope to show it to my kids!

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