Observation More About Mercury or Venus Evening or Morning Stars

Mercury and Venus are observable like a morning, or a evening star only. That is due to that those are inferior planets, which are located between us and the Sun. That way, as seen perspectively, they never can get distant from the Sun to the point they would become planets visible against the starry night background (for more about the apparent motion of inferior planets check our tutorial 'Planets Apparent Motion')

Venus seen like a evening star! picture site 'Amateur Astronomy'

Mercury is seen reappearing like a evening star 12 days after a superior conjunction and like a morning star 5 days after a inferior conjunction. Mercury is never more than 28 degree farther from the Sun. Venus is like a evening star 35 days after a superior conjunction and like a morning stars 6 days after its inferior conjunction. Venus is never more than 48 degree farther from the Sun. That difference which exists between when Mercury or Venus is reappearing like a evening star compared to when they do like a morning star is due to celestial mechanics. That is pecularly observable as far as Venus is concerned. Venus is passing from a morning, to a evening star after a superior conjunction. At that occurrence, Venus is on the other side of the Sun relative to the Earth, by some 160 million miles (257 millions kilometers). It thus looks like it has a apparent motion slow, and with the same direction than the one of the Sun, which is eastwards. Those factors as combined make that Venus is remaining a long time in the morning, and then evening glare of the Sun. Venus eventually appears like a evening star, low, West and it then takes several weeks before it fully reaches it status of a evening star. By a inferior conjunction, at the opposite, when Venus is passing from a evening to a morning star, it is located between the Sun and the Earth, by some 25 millions miles (40 million kilometers) only! Its apparent motion thus is swift, and opposed to the one of the Sun, westwards that is. Venus, due to that, is reaching earlier its full status of a morning star, along a duration of one or two weeks only. As it is closer to us further, it also is shining more brightly more rapidly. When a inferior planet reappears like a evening star, it does under the form gibbous, mostly. When reappearing like a morning star, it does under its maximal crescent form

In terms of what phase displayed during a apparition, Mercury and Venus also, are waxing when a morning star and waning like a evening one. Mercury generally, is best seen in the northern hemisphere like a evening star by spring, and a morning star by fall -and the reverse in the southern hemisphere. A a greatest elongation, Venus may rise or set up to 4 hours and half before or after the Sun. At a greatest elongation, Mercury may rise or set up to 2 hours before or after the Sun