Mercury can only be observed from Earth for a brief time in the pre-dawn and post-dusk sky. The orbits of the inner planets shown in the chart above explain why we cannot see them at night. Note that Mercury will always be located in the sky no more than 28 degrees from the Sun, and Venus is always within 47 degrees.. When the sun is visible, then we cannot see planets or stars with the naked eye, but we can see them when the Sun is just below the horizon either before sunrise or after sunset.