1. What is a planet? B R Sitaram Zeal Education

2. What is a planet? ► Books give various definitions: ► A celestial object that is circling the Sun: how do we know which objects circle the Sun and which do not? After all scientists and philosophers believed till just 500 years ago that planets revolve around the Earth

3. What is a planet? ► Stars are self-luminous, while planets only reflect light from the Sun. How do we know this? It is only in the last 150 years or so that we have been able to study the light from planets and stars using spectroscopy to realize that stars are hot objects emitting light but planets are cold.

4. What is a planet? ► Planets are wanderers ► This is the classic definition of planets; in fact, the word planet means the wanderer in Greek! ► But what do we mean by wanderer? ► If you look at the night sky, you will see that all objects move!

5. What is a planet? View of the sky at 8 pmView of the sky at 11 pm Notice how stars appear to be moving to the right

6. What is a planet? View of the sky at 2 amView of the sky at 5 am Notice how stars appear to be moving to the right

7. What is a planet? ► Why does everything in the sky move? ► Because the Earth is rotating!! ► The stars do move, but extremely slowly, over hundreds of years! ► So what do we mean by a wanderer?

8. What is a planet? ► To understand, let us freeze the motion of the stars (or stop the rotation of the Earth!) ► How? Sidereal period: After 23 h and 56 minutes, stars are back in the same position! ► Take snapshots after every 23 h and 56 minutes! ► Video shows Venus movement relative to the stars

10. What is a planet? ► You can clearly see that Venus (and other planets!) are moving WITH RESPECT TO THE STARS! ► That’s why a planet is a wanderer!! ► But wait, the motion appears to be nice and smooth. ► Wandering should mean some other kind of motion, right?

11. What is a planet? ► If we study the motion of planets over long times, like the ancients did, you find strange movements! ► Here’s a video that shows “retrograde motion”: Venus appears to move backwards! ► Again, the video has been made from snapshots taken after every 23 h and 56 minutes

13. What is a planet? ► If we had continued taking snapshots for another month, we would have seen Venus going forward once again! ► Other planets also show similar motion: the image shows the motion of Mars in 2003.

14. What is a planet? ► So, planets do wander in the sky, relative to the stars! ► Of course, not all objects that wander are planets: comets, asteroids, minor planets all wander. ► For classical astronomers, planets were the most prominent objects to be seen regularly: comets, the other kind of bright object, were rare. ► Note: The images were captured using SNARF and SKYGLOBE software.