Retrograde Motion

Everyone knows that over the course of a day or night, objects in the sky generally move east to west because of the Earth's rotation. But when you look at the position of the Moon and outer planets compared to the background stars, they actually move west to east. This is the apparent motion of the Moon and planets.

For example, if Mars and a star are next to each other in the sky, the next night, Mars will have moved farther east compared to the star. We call this motion prograde or direct motion.

However, there are times when the planets will have move west with respect to the background stars. This is called retrograde motion.

It is because of this retrograde motion that Ptolemy introduced the concept of epicycle to his geocentric model of the solar system. There would be not other way for retrograde motion to be explained if the planets orbited around the Earth. It wasn't until Copernicus and later Kepler that retrograde motion could be easily explained without the use of epicycles.

A good way to imagine retrograde motion is two cars on a highway. In general, a car in the fast lane will be going faster than a car in the slow lane. As the car in the fast lane approaches the slower car, both cars are going in the same direction. But as the faster car passes the slower car, to occupants in the faster car, the slower car seems to moving backwards with respect to the background trees, mountains, signs, etc.

In planetary motion, the planets closer to the Sun move faster than the planets farther away. So as Earth approaches the position of Mars, we see Mars moving west to east with respect to the background stars. However, as we get "close" to Mars, its motion seems to change direction and we have retrograde motion.

Drawing of how retrograde motion works. Credit: Wikipedia User Rursus

The apparent motions of Venus and Mercury are a little more complicated than that of the outer planets because they are closer to the Sun than Earth is. I'll discuss this in the next post.

Annular Solar Eclipse on February 26, 2017

Today, if you are lucky enough to live in the southern hemisphere, there will be an annular solar eclipse. It is going on right now.

https://www.timeanddate.com/eclipse/solar/2017-february-26

Planetary System around TRAPPIST-1

On February 22, 2017, scientists announced that there were at least seven terrestrial planets orbiting around TRAPPIST-1. Three of them are within the habitable zone. What does this mean for us?

We should learn a little about TRAPPIST-1. It is an M8V star in the constellation Aquarius, approximately 39.5 light-years (12.1 parsecs) from Earth. As an M8V star, it is smaller and cooler than Earth. In 2015, the first three Earth-sized exoplanets were discovered around the star. It wasn't until the recent announcement that they confirmed three of the planets were in the habitable zone. The habitable zone is the zone around a star where liquid water can be found. Look at the post about Earth-like planets to see why both are important.

For a star like TRAPPIST-1, the habitable zone is much closer to the star than it would be for Earth. It is likely that life might be on these planets? No, because these planets are so close to the parent star, and TRAPPIST-1 is very active, if these planets had any atmospheres at one time, they have likely been stripped away. If any life exists, it will be primitive in nature, one-celled lifeforms, if any.

Image courtesy of NASA

As shown in the above image, these are all considered terrestrial planets. They have radii and densities comparable to Earth. They are made up of mostly refractory elements.