Lesson Objectives:- The planets
- Apparent retrograde motion
- Stellar parallax
5 planets are easy to find with the naked eye - Mercury, Venus, Mars, Jupiter and Saturn. Mercury is rarely visible since it is so close to the Sun, but you can occasionally catch a glimpse of it just after sunset or just before sunrise. If you see a bright 'star' in the early evening or early morning, it is probably Venus. Jupiter, when visible, is the brightest object in the sky besides the Moon and Venus. Mars can be recognized by its reddish color. Saturn, while highly visible, can look similar to other stars so it can be helpful to use a star chart to identify it.
Something that can also help you to identify planets is the fact that planets do not twinkle as much as stars.
For thousands of years, ancient peoples believed in an Earth-centered universe, but as they observed the planets that were visible to the naked eye, there was one particular mystery that they had difficulty explaining.
The stars, the Sun, and the Moon always appear to rise in the east and set in the west, due to Earth's rotation. Planets, however, during the course of the year, will sometimes reverse direction and go westward instead of eastward, in what is referred to as apparent retrograde motion.
If you look at the diagram on the right, you can see what causes this. If you visualize yourself as the green dot walking quickly around the Sun and a friend as the red dot walking more slowly, there is a point where your friend will appear to move backwards - when you catch up to and pass her.
This is a simple concept to explain if you know that the Earth orbits around the Sun, but since the ancient Greeks believed in an Earth-centered universe, they were forced to come up with some very complex explanations for apparent retrograde motion.
Stellar Parallax occurs where a star appears to shift in position when we see it from different points in Earth's orbit.
Stellar parallaxes are not detectable to the naked eye because stars are so far away, and even with the most powerful telescopes, parallax can only be measured for the nearest stars. A star's parallax can be used to calculate its distance from Earth using trigonometry.
The ancient Greeks rejected the idea of stellar parallax because they assumed the Earth was stationary at the center of the universe, and also because they did not believe the universe could be big enough for parallax to be undetectable to the naked eye. However, the invention of powerful telescopes later proved that the Universe is so much bigger than our solar system and galaxy alone.