Venus is sometimes called the morning star or the evening star because very often it is visible either as the first bright star in the sky during twilight, or the last bright star in the sky at dawn. Both of these strange but predictable occurrences can be explained by Venus’s position in the solar system relative to Earth.
Venus is, of course, a planet (like Earth), not a star (like the Sun). It “shines” in the sky for the same reason that the Moon does: Sunlight is reflecting off Venus (or rather, the thick clouds of Venus’s atmosphere). It is also the closest planet to Earth, at just 26 million miles away, which is why it appears to be the brightest star in the sky.
More importantly, however, Venus is also significantly closer to the Sun than Earth is, and the relationship between the two planets’ orbits is the reason why Venus sometimes appears to be a “morning star” or an “evening star.” NASA explains why. Venus – as well as Mercury, which is much smaller – has an orbit inside Earth’s orbit, so it always appears to be relatively close to the Sun. This creates four possible circumstances.
First, Venus may be located between the Sun and Earth, known as an “inferior conjunction.” In this case, it cannot be easily seen against the background light of the Sun. This is similar to what occurs during the new moon phase of the lunar cycle: The Moon passes between the Sun and the Earth, so that no light reflected from its surface reaches the eyes.
Second, Venus may be located on the far side of the Sun, known as a “superior conjunction.” In this case, of course, it also cannot be seen.
As it moves between the two conjunction points, however, Venus becomes visible, and because of its proximity to the Sun, it becomes visible in regular, predictable ways. First, as it moves out from behind the Sun, it seems to trail slightly behind the Sun in its path through the sky, meaning that it becomes visible to the naked eye at sunset. As it continues along its orbit, it appears to set later and later at night, until it reaches a maximum point or maximum “elongation.” At that point, Venus’s evening appearances shorten again as it moves between the Earth and the Sun, and therefore gets lost in the background light of the Sun. During this part of the cycle, Venus is usually the brightest star visible after sunset, and is referred to as an “evening star.” At maximum elongation, it can also be visible in daylight.
Lastly, Venus becomes visible again as it moves out from between Earth and the Sun and becomes visible again. This time, however, it appears on the other side of the Sun – which is to say, it occurs just before the sunrise in the morning, rather than just after the sunset at night. Venus now appears to have steadily longer periods in the pre-dawn night sky, before it reaches another point of maximum elongation. Then, it begins to pass back behind the Sun and disappear again. During this part of the cycle, Venus is the last bright star visible in the sky at dawn, and is referred to as a “morning star.” Again, at maximum elongation, it can also be visible in daylight.