A Zero Shadow Day is day when the Sun will be directly overhead at local noon. It can happen only in locations between the Tropic of Cancer and the Tropic of Capricorn. The Sun never reaches this point on any day in locations north of the Tropic of Cancer and south of the Tropic of Capricorn. Between the Tropics, this happens twice a year. You can read more about Zero Shadow Day here.
Usually, we will carry out experiments where we place objects like cylinders and blocks and observe that there is no shadow at local noon.
For a change I decided to use a different cylindrical shape – my Newtonian Telescope – and instead of looking at the shadow, we will look straight up at zenith – WITH A SOLAR FILTER COVERING THE ENTIRE APERTURE AND ALL USUAL SOLAR PRECAUTIONS. So I covered the aperture with a solar filter, connected the camera and left my Newtonian telescope aperture up – checked the level to ensure it was more or less pointing straight up. (using my mobile leveling tool)
If the Newtonian telescope is properly level, then the Sun will drift into view of the camera when the Sun is about to reach zenith. Check out the image below which was taken by the setup above when the sun reached zenith.
You can catch the video below.