Why Venus is known as the morning and evening star?

Venus is the second planet from the sun and is the brightest celestial body in Earth’s night sky after the moon. Venus shines so brightly that it is also referred as the brightest star seen from earth before the sun rises or after the sun sets ( therefore also known as morning and evening star ). The Venus shines in the sky cause of the same principle as Moon i.e. Venus reflects the light from the Sun.

To further understand the topic, we need to understand the ‘’Synodic period’’ of Venus.

*Synodic period — The time required by an celestial body ( Moon, Planet, Artificial Earth satellite ) to return to the same position or approximately the same position relative to the sun as seen by the observer on the Earth.

The Synodic period of Venus is 584 days which can be sub divided into 263 days western elongation ( morning star ), 50 days absent, 263 days eastern elongation ( evening star ) and 8 days absent.

The figure shows the Sun, the Earth and the Venus at three positions in its orbit.

When Venus is westerly elongated at position V2, an observer on Earth at position O1 would see Venus rising while the Sun is still below the horizon. The Sun would appear to rise when the Earth rotates further till the observer is brought round to position O2. Venus would be visible above the eastern horizon for a few hours before the sun rises (as it rises before the sun, it will also set before the sun and therefore will not be visible in the evening after sunset). During this period, Venus is known as the morning star as it is only visible in the morning before sunrise . Because of the brilliance of the sun, after the sunrise, Venus would not be visible to the naked eye.

When Venus is easterly elongated at position V3, an observer on Earth at position O3 would experience sunset. Venus would still be above the horizon and would be visible to the naked eye till it sets when the Earth rotates further, till the observer is brought round to position O4. Venus would be visible above the western horizon for a few hours after the sunset (as it sets after the Sun, the next morning it would also rise after the sunrise and therefore would not be visible during the day due to the brilliance of the Sun). During this period, Venus is known as the evening star as it is only visible in the evening after sunset.