Zero Shadow Day

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

{ZSD season 2017 for India begins 10th of April when people Kanyakumari
will observe the first ZSD.
Click here for other places in India  for the ZSD date. }

Well this is exactly what can be experienced by those at a tropical latitude – between the two tropics – the tropic of Capricorn (-23.5°) and the tropic of Cancer (+23.5°).  For them, twice every year there comes a day when the Sun passes directly overhead at noon. Shadow falls directly below and seem to have disappeared.  The point directly overhead is called Zenith.

For observers on that latitude – it is a Zero Shadow Day (ZSD)

For an observer in the northern hemisphere, between winter solstice (Dec 21/22 ) and vernal equinox (March 20/21) the shadow of a vertical object at noon always points ‘north’. After ZSD the shadow at noon points south.  After the summer solstice (June 20/21) once again there is a ZSD and after this day then the shadow at noon points north again.

Exactly opposite happens for those in the southern hemisphere. For them the ZSD is between autumnal equinox (June 22/23)

We may note here that those north of +23.5° latitude will never see ZSD and for them the shadow at noon will always point due north and for those south of -23.5° latitude the shadows at noon will always point due south.

Observing ZSD

The phenomenon itself is quite enjoyable and it carries a high value of edutainment.  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 then to reappear again on the other side.

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.

The event can be used to discuss the curvature of the earth and how Eratosthenes found the circumference of the Earth.

ZSD – when and where 

IITian Alok Mandavgane (MTech in Mathematics and Computing ) is a Android and Developer, Web Developer.  Alok has developed an excellent app and page for finding ZSD for a given location on the Earth. Please visit  this link.  He is a serious amateur astronomer and  Joint Secretary at Aryabhat Foundation.

For those who want some quick ready info  – Click here.

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
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.

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 height. 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.

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, (IUCAA) became quite very popular with the press and general public.

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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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