Question

If earth has no atmosphere than what chang would be obserb in the lenth of day

Answer 1

There will be only about a 4 minute cut down in the length of the day because, the first 2 minutes and the last two minutes of the rising an setting of the sun is just am illusion . As the sun rises the width of the atmosphere (being a physical medium ) bends the light emitted by the Sun (observed in a prism  by Issac Newton) hence there is an illusion of the Sun in the sky for 2 minutes during Sun rise and for 2 minutes during Sun set.