Observations by SPACE in INDIA


What is a Gnomon?

A "stick'' on a sundial that casts a shadow on the face of the sundial. The position and/or length of the shadow gives the time and date. The Black rod given to you is the gnomon in this experiment.

I am not able to fix my Gnomon base to the board.

Please try thumb tacks, common if you have a soft board like rice husk or thermocole. If you are using ply use a thin nail but do not hammer it in all the way. That will make it difficult to pull it out and retrieve the paper

I do no know how to make a plumb line.

Take a piece of twine about 25 cms. Tie a nut at one end. Your plumb line is ready!

I do not know how to make the gnomon vertical.

After attaching the gnomon base to the board hold the plumb line to hang freely next to the gnomon. If the string and the gnomon are parallel then your gnomon is vertical. Otherwise slightly tilt the board by placing thin cardboard below the board on one side till the two are parallel. Always wait for the plumb line to be steady before judging whether the two are parallel. This is an important adjustment

I do not know how to measure the length of the shortest shadow.

After removing the paper from the board place it on a table. Join all the pencil marks made of the tip of the shadow. This will form an arc. From the centre of the Gnomon measure the distance to the point on the arc that is shortest. You may have to measure it at two or three points and take the minimum value. Express this distance in mm

I do not know how to construct a similar triangle and measure the vertical angle.

Take the average length of the Gnomon that you have measured in mm (H)
Draw a vertical line equal to 2H mm in length. Take the length of the shortest shadow measured in mm (L). Draw a line equal to 2L mm in length and perpendicular to the vertical line at one end. Use geometry construction method for making it exactly perpendicular to the vertical line. The scale factor used in the above example is 2. You can use other convenient scale factors. Join the ends of the two lines to form the similar triangle. Measure the vertical angle with a protractor.

very rich website (with many onward links) by Chuck Bueter

Informative and activity listed on the site.



Calculate the sky at your area during the transit of Venus.

Explaination of Phenomena observed during the Transit of Venus.



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