Why does a spherometer have three legs?​

Recall from geometry that three points determine a plane, so the three legs will always touch the glass. ... One complete turn of the central screw of the spherometer raises or lowers the center point a set distance which is determined by the size of the thread of the screw and the radius of the disc.

Explanation:

spherometer is a device used to measure curved in surface

it have 3 legs which form equivalent triangle.

geometry says that 3 point determine a plane that's why it have 3 legs

one "light year" is the distance that light travels, through vacuum, in one julian year.   that's 365.25 days, of 24 hours each.