Close to Earth, Mars to look extra big and bright

Saturday, Oct 10, 2020 4:03 AM

Greetings, Stargazers.

This is definitely the season for Mars, because this month, Mars will be closer to Earth than at any time in the next 15 years.

Mars reaches opposition when it and the Earth are in the same direction from the sun. On that date, it will be crossing the meridian and reaching its highest point in the sky at local midnight. This happens about every 26 months, and the next opposition will occur Oct. 13. A Martian year is 687 days, and its most recent perihelion, when it makes its closest approach to the sun, was on Aug. 3. Whenever an opposition happens near perihelion, Mars will be extra close to Earth, so appear extra big and bright.

Many readers will be familiar with the recent term supermoon, which happens when a full moon occurs at perigee, or its closest approach to Earth. This year, we get a super-Mars. The next time this will happen is 2035. From some quirks of both Earth and Mars having elliptical orbits, Mars was actually slightly closer last week on Oct. 6, but all month, Mars will appear bigger and brighter in the sky than it will for quite a while.

But “big” is a relative term, and no matter how close we get, planets always appear tiny through a telescope. The angular size of things in the sky is given in arc seconds, arc minutes, and degrees. There are 60 arc seconds in 1 arc minute, and 60 arc minutes in 1 degree. If you hold your hand out to arm’s length, your pinky finger will take up about 1 degree of your field of view. The thickness of your fingernail would be about 1 arc minute. This is about the limit of resolution of the human eye. In other words, if you have 20-20 vision, you should be able to just barely distinguish two pinpoints if they were separated by 1 arc minute.

To add confusion to describing the size of things, the symbol for arc seconds is the same one used for inches, and the one for arc minutes is the same one used for feet. So when you see that at this opposition, Mars will appear 25” in diameter, that means 25 arc seconds, and not 25 inches. Maybe this is the story behind some social media myths that Mars will sometimes look as big as a beach ball.

The small size of planets is precisely why it is hard to get high-resolution photographs from Earth. Most of the time, turbulence in the atmosphere will limit what you can see through the eyepiece and also the resolution of photographs to 1 or 2 arc seconds. That would mean that the best Earth-based picture of Mars could be at most 25 pixels across. This atmospheric limit is called seeing, and was one of the primary motivations for launching the Hubble Space Telescope.

However, if you are an experienced observer through a telescope eyepiece, you know that sometimes you get lucky. Sometimes, for a fraction of a second, the atmosphere will be extra still, and you will briefly get an extra-clear image of what you are trying to see. This is the basis for some modern photographic techniques, appropriately called lucky imaging, that let amateur observers take images that are significantly better than anything that was possible only a couple of decades ago. Using what is essentially a webcam, hundreds of images can be taken in a few seconds, and then computer software can quickly pick out only the best ones to average together. With this approach, it is possible to get images of bright objects like planets that show details only limited by the size of the telescope.

This month

Mars is rising in the east soon after sunset. This is one of the few months that it will be brighter than Jupiter. Jupiter and Saturn still make excellent observing targets in the southwestern sky, and over the next couple of months, they will be getting closer and closer to each other. In December, during a rare, extra-close conjunction, they will only have a few arc minutes of separation.

The Orionid meteor shower is the result of debris from Halley’s comet. It is a fairly reliable producer of October meteors and the peak will be around Oct. 21. Look for these meteors after midnight when Orion is above the horizon.

Charles Hakes teaches in the physics and engineering department at Fort Lewis College and is the director of the Fort Lewis Observatory. Reach him at hakes_c@fortlewis.edu.