Unit 1 : Activity 1 : Sunrise, Sunset, Star-rise, Star-set : Crescent Venus : The Wanderers in our Sky : Unit Exam

The Wanderers in our Sky

Time lapse pictures from planetarium programs or a real planetarium over the course of a year show the following movement of some objects among the stars.

We know today that these objects are the other planets in our solar system. Although they didn't have the advanced tools we have access to today, ancient astronomers (2000 to 4000 years ago) were very familiar with these wanderers in the sky. They found ways to describe and even to predict their motions.

We say that the planet moves prograde (forward) most of the time, when it travels from West to East along the ecliptic. It is important to remember this motion is relative to the stars. The motion is referred to as retrograde (backwards) when the object is going the other way. One of the first patterns that the ancient astronomers would have noticed is that the retrograde motion for Mars (and for Jupiter and for Saturn) always occurs when the planet is near opposition, that is, when it is highest in the sky near midnight. You probably noticed this odd motion yourself doing Activity 1. If you didn't, go back and look at the motions of the planets again and see if you can spot this strange looping. (This might be described as the "Ooops I forgot my keys! Wait, here they are in my pocket." motion.)

Synodic periods of planets.

Mars goes through retrograde motion when it is near opposition. This is one of the special arrangements or configurations that Mars can be in. In Unit 3 you'll have a chance to learn about some other special configurations of the planets and see how these can be useful.

At opposition Mars and the Sun are in opposite directions as seen from Earth.

As a result, Mars is highest in the sky when the Sun is lowest (at midnight).

From one opposition for Mars to the next is just over 2 years. We call this the synodic period of Mars. The synodic periods of the planets are given in this table.

Planet Synodic period, years Sidereal period, years
Mercury 0.3173 0.24085
Venus 1.599 0.61521
Earth --- 1.0000
Mars 2.135 1.88089
Jupiter 1.092 11.8653
Saturn 1.035 29.6501
Uranus 1.012 83.7445
Neptune 1.006 165.951
Pluto 1.004 247.687

IIf the synodic period is the time from one arrangement (opposition) to the same arrangement of the Earth, Sun, and Planet, then what is this "sidereal period" in the table? From the point of view of an observer on Earth, the sidereal period is how long it takes for the planet to end up in the same part of the sky, against the same background stars. In our modern view of the solar system, with the planets orbiting the Sun, the sidereal period is the same as the orbital period, or the individual year, of the planet. So in this case, Saturn's sidereal period or “year” is 29.6501 Earth years.

Do you notice any patterns in this set of numbers? Check out this "Little Extra" to find out how the sidereal and synodic periods are related.


Congratulations! You have completed the first unit of Evening Star. Once you feel comfortable with the material, please proceed to the end of unit quiz.