This spring will bring us the best view of Mars in seven years.
The best views always occur when the planet is at opposition — when it appears to be opposite the sun as seen from Earth. To see surface features, you will need at least a six-inch telescope.
Mars and Earth travel around the sun at different rates. It takes the Earth about 365 days to make the trip, but it takes Mars longer — about 687 Earth days. At opposition, Mars, the Earth and the sun are all in line, with the Earth in the middle. The planets are therefore closer to each other during this period than at any other time in their orbits.
Mars will be at opposition on April 8, and our best views will be when it is closest to Earth, during the middle two weeks of April. But Mars comes to opposition every 26 months, so why are these the best views in seven years?
All oppositions are not created equal.
The Earth is in an almost circular orbit around the sun, but Mars is not; the distance between the sun and Mars varies by almost 31 million miles. If opposition occurs when Mars is closest to the sun, the distance between Earth and Mars is 31 million miles less than if it occurs when Mars is farthest from the sun. The distance at opposition on April 8 is closer than those in the past seven years.
As the planets get closer together, the apparent size of Mars will increase. Astronomers measure the size of an object in the sky in degrees or fractions of degrees of arc; 60 arcminutes equals one degree of arc and 60 arcseconds equals one arcminute. During February Mars will grow from 9 arcseconds to 11.5 arcseconds, and by the end of March it will be 14.6 arcseconds. At its closest approach in April, its maximum size will reach 15.1 arcseconds. It is this increase in size that reveals Mars in more detail, giving us the good views.
Even this large size is not the best possible view of Mars. In July of 2018 it will be closer and reach 24.3 arcseconds.
As Mars gets closer to Earth during February and March, it will also get brighter. By the end of February it will be significantly brighter than the bright star Arcturus, seen far to the planet’s upper left. Mars will continue to brighten during the month of March and at opposition reach a maximum brightness of magnitude minus 1.4.
Where we find Mars in the night sky also changes. There are three reasons for the changes in location.
First, the Earth is moving in its orbit around the sun, causing the sky to appear to move a little to the west each night. With this westward movement, the stars and planets rise earlier and appear higher in the sky than the night before at the same local time.
At the beginning of February Mars rises in the Southeast at about 11 p.m. By the end of February it will be rising at 9:30, and by the end of March it will rise at 8 p.m. At opposition on April 8 Mars will be due south at midnight local time.
The second reason for the change in location is that Mars is moving eastward in its orbit around the sun. If you watch Mars during the month of February, it will slowly move to the east relative to the stars, but by the first of March it will appear to stop this eastward motion and start moving west.
This westward or “retrograde” motion is caused by the third factor for determining Mars’ position in the sky: the Earth is passing Mars. Mars is not actually changing direction — it is continuing in its orbit around the sun — but because we are passing the planet, it appears to move backward from our point of view on Earth. This is just like when you are driving down the road and pass a slower car: both cars continue in the same direction, but as you pass the car, from your point of view it appears to move in reverse.
All planets show this retrograde motion sometime. By odd coincidence, in an eight-day period between Feb. 27 and March 6, four planets and the two brightest asteroids will appear to halt their motion among the stars: Mercury on Feb. 27, Ceres and Mars on March 1, Saturn on March 3, Vesta on March 5 and Jupiter on March 6.
So let’s hope we get at least a few clear nights in the next few months so that we can watch Mars move in the sky and get brighter from night to night. Especially if you have access to a telescope, you might get some great views of Mars.
Marty Scott is the astronomy instructor at Walla Walla University, and also builds telescopes and works with computer simulations. He can be reached at firstname.lastname@example.org.