What's New: July 2018

Mars Comes Close

Earth goes around its star in a year; Mars, moving at a more leisurely speed in its more distant orbit, takes about 1.88 years. Every 26 months or so, Earth overtakes Mars. There comes a moment when the Sun, the Earth, and Mars are all in a line, before our planet passes Mars and speeds on. Such a line-up is referred to as an "opposition." The term originates in the fact that, from our point of view on Earth, Mars and the Sun lie in directly opposite directions in our sky. It follows that, on such occasions, Mars will be seen to rise as the Sun is setting, and will set at sunrise; it will, in other words, stay up all night.

It also follows that the two planets are closest at opposition. All other things being equal, it is then that Mars will appear the largest and brightest. Such a Mars opposition is coming up this month – on July 27, to be exact.

It turns out, however, that not all Mars oppositions are equal. Earth's orbit around the Sun is not precisely circular, and that of Mars is substantially eccentric. So even though Mars oppositions do tend to occur every 26 months, some result in closer encounters than others; this repeats in a 15-year or 17-year cycle.

Martian close approaches 2003-2018

The opposition of 2018 will be a very close one. The last time Earth and Mars had such a close encounter would have been in 2003; at that time the distance between the planets was less than it had been for almost 60,000 years! This July, they will be almost as close.

And how close is that?

Because of the eccentricity of the orbits of the two planets as referred to above, the moment of closest approach occurs, not precisely at the moment of opposition, but several days later. On July 31, Mars and Earth will be separated by 35.78 million miles. A radio transmission from the Opportunity or Curiosity rover on its surface – traveling at the speed of light - takes nearly three and a quarter minutes to reach us.

Mars