Why stars differ in brightness, space telescopes and a human sundial

A sundial tells the time with a shadow. It has an upright part called the gnomon, and a flat part with hour marks. As the Sun seems to cross the sky, the shadow of the gnomon moves across the marks. People used shadow clocks and sundials for thousands of years, including in ancient Egypt. In our human sundial, a person is the gnomon. Choose a flat, open, sunny place and mark a standing spot with chalk. Find north with a compass or with a grown-up. At each hour the person stands on the spot, and a partner marks where the shadow of the head falls, labelling the mark with the hour. The next day, a person stands on the same spot and reads the time from the marks. The sundial must stay exactly in one place, and the person must stand on the same spot. If it is moved or turned, it will show the wrong time. The marks also match only the time of year they were made, because the Sun’s path changes with the seasons. Never look at the Sun: look at the ground.