Most of the people whenever to look towards night sky; they try to find out the brightest star in the sky. If you also had tried the same then you must have observed that the brightest star in the sky can be viewed only during evening or morning.
What is morning and evening star?
The planet which has orbit smaller than the orbit of Earth is called the inferior planet. The planet Mercury and Venus are the only inferior planets. The Mercury and Venus both behave similarly so we will deal with them in general as inferior planets. Because of the smaller orbit of the inferior planet, they usually appear in the region around the Sun and cannot go opposite to the Sun in the night sky. The closeness of inferior planet to the Sun forces them to act as morning and evening stars.
In the ancient time, people believes that the evening and morning stars are different objects. When Venus was a morning star then it was known as Phosphorus or Lucifer and when it was an evening star then it was known as Hesperus.
Let us look in detail how does the inferior planet act as morning and evening star?
Different positions of Mercury and Venus around the Sun.
The inferior planets revolve around the Sun in elliptical orbits and the distance of inferior planets from the Earth varies according to the different positions in orbit.
When the inferior planet is on the opposite side of the Sun then it is at the farthest distance from the Earth. This position is called Superior Conjunction. At Superior Conjunction, the planet is hidden behind the glare of the Sun and cannot be observed.
Now the inferior planet moves towards the left or towards the Eastern sky as seen from the Earth. In doing so, the separation between the inferior planet and the Sun increases as seen from the Earth and become largest at the position called the Greatest Eastern Elongation. At Greatest Eastern Elongation, right or Western half of the face of the inferior planet is illuminated by the Sun as seen from the Earth.
As the planet proceeds on its orbit, the distance between the inferior planet and the Sun appears to decrease and becomes zero when the planet passes in front of the Sun. At this position, the inferior planet is at the shortest distance from the Earth. This position is called the Inferior Conjunction. At Inferior Conjunction, it is not necessary that planet passes exactly in front of the Sun’s disc due to the inclination of orbit of the inferior planet with respect to the orbit of Earth. But whenever the inferior planet passes exactly in front of the Sun’s disc, the phenomenon is called the transit. Otherwise the planet is completely lost in the glare of the Sun and cannot be observed.
Now the planet moves towards the right or western sky as seen from the Earth and the distance between the inferior planet and the Sun appears to increase and become largest at the right most position called Greatest Western Elongation. At Greatest Western Elongation, the left or Eastern half of inferior planet is illuminated by the Sun as seen from the Earth.
After this, the inferior planet again moves behind the Sun and reaches the position of Superior Conjunction and the cycle repeats again. After knowing about the different position, we can discuss the role of Venus and Mercury as morning and evening stars.
Mercury and Venus as Evening Star
After the position of Superior Conjunction, the inferior planet moves towards the Eastern sky and reaches the Greatest Eastern Elongation and then goes to Inferior Conjunction. During this Eastern half of the orbit, the planet follows the Sun as Earth rotates from West to East. It means during rotation of Earth, the Sun moves towards West and sets in the Westerns sky in the evening but the inferior planet is still behind the Sun in the Eastward direction and it will set in the western sky after some time. So the inferior planet will keep on glowing in the western sky in the evening after the Sunset and known as the evening star. The highest position in the western sky in the evening will correspond to the Greatest Eastern Elongation.
Mercury and Venus as Morning star
After the Inferior Conjunction, the inferior planet moves towards the western sky for the position of Greatest Western Elongation and then towards Superior Conjunction. During this Western half of the orbit, the Sun follows the inferior planet, means as the Earth rotates the inferior planet moves from East to West but the Sun is lagging behind in the Eastward direction. So the inferior planet will rise in the morning in the East much before sunrise and keep on glowing for some time and it will lose in the day light with the sunrise. At this time the inferior planet is known as a morning star and the highest position in the eastern sky in the morning just before sunrise will correspond to the Greatest Western Elongation.
Observation of Venus and Mercury
As the orbit of Mercury is smaller than the orbit of Venus, the Greatest Elongation is also smaller in the case of Mercury which will result in the smaller height of Mercury above the horizon as compared to Venus. So the Venus can be seen much higher in the sky as compared to Mercury.
Mercury acts as Evening and Morning star much frequently as compared to Venus because orbital period of Mercury is just 88 days as compared to 224 days in case of Venus.
If you want to distinguish whether the morning or evening star is Venus or Mercury, you need to look for brightness and height above the horizon. The brightest one is Venus but the height may not be helpful each time.