Caoimhín's Content

As you may have seen in some of the recent posts here, Mars is beginning to be visible in the morning sky, but we’re going to have to move to the end of March and beyond before it reaches a good position for observation, at least those of us in higher latitudes. Starting right on the 1st of March, close to the horizon we can see Venus, and if you take a close look, Mars would be visible as well. This close to sunrise, I can’t recommend actually using a telescope to take a closer look. The Red Planet is pretty much invisible if you are in a city with light pollution, and even if you are in a darker countryside location, Mars is so low in the sky that it is very easily blocked out. If you were to take a closer look at Mars when it is close to sunset or sunrise, you would see almost a Full Mars. The closer Mars is to being behind the Sun, the closer to full it will look, but the harder it is to observe. Whenever you take a look at Mars, there’s a chance that you’ll see one or both of Mars’ moons, Phobos and Deimos, but they are so small that they are always tricky to spot, they are both very similar to an asteroid in size and shape.

When conditions are better, Mars is a fantastic planet to take a closer look at. With a very thin, generally cloud free atmosphere (bar the occasional sandstorm), the surface of Mars is visible quite clearly. Mars rotates at almost the same speed as the Earth, just a little bit slower. This means that if Mars is up all night, or at least for a lot of the night, you can see it turn over the course of a few hours. The slightly slower rotation also means that you’ll see a slightly different view of Mars each night, if you go out at the same time each night. Every 24 hours, when the Earth has turned the whole way around to face Mars again, Mars has rotated just a little less. This means that over the course of a few nights, you’ll see a whole new side of Mars as it’s rotation lags behind ours. Mars’ orbit is also a little slower than ours, so what time of year we see it in the morning will also vary from year to year. This year, we won’t be able to observe Mars for very long, it simply isn’t in darkness for long enough in the morning. Even at the end of March, from the city Mars will remain out of view, though it will start to be visible under good conditions.

This is despite Mars continuing to push along in it’s orbit and appear further and further from the Sun. As we are moving back towards summer time the days are getting shorter, and that includes an earlier sunrise. Mars may be further from the Sun, but the Sun will continue to light up the sky earlier and earlier, pushing out the date at which it will become easy to observe. By the time Mars is visible, Saturn will appear further from the Sun from our perspective. Saturn will appear from behind the Sun and overtake Mars in our sky. Saturn of course is moving around the Sun slower than Mars. It’s the Earths motion around the Sun that brings Saturn back into view, Mars appears to move slower in the sky here because it is moving away from us. Even though Mars is orbiting the Sun slower than the Earth, it is still moving around the Sun in the Same direction as we are. This means that the Earth needs to catch up to Mars. We are moving towards Mars, bringing it out from behind the Sun, but Mars is continuing to move away from us, which has the effect of slowing down it’s apparent motion in the sky. Jupiter is leaving the evening sky as we leave March, but in almost no time at all, just a few months, it will have popped back up in the morning sky. This will give us a wonderful vista in the morning as we move into summer this year, with Jupiter and Mars, the Moon and Saturn all on a row. just a few months later and Jupiter has overtaken Mars as well, for the same reason as Saturn. Even though Jupiter is faster than Saturn it is still significantly slower than Mars.

In March, Mars is visible in the morning, ahead of both Jupiter and Saturn. By the end of the year, both Jupiter and Saturn will have popped up in the morning and will appear to overtake Mars, appearing in the evening when Mars is still stuck in the morning sky. As Saturn and then Jupiter pass the South at midnight, or the North if you are in the Southern Hemisphere, they will reach opposition. This is when they are at their closest to us, and just like the Full Moon, they are on the opposite side of the Earth to the Sun, they will stay up all night and be at their brightest and easiest to observe. Even late this year, Mars is only starting to reach due South at about 3 o’clock in the morning. We won’t get to see Mars at opposition this year at all.

When the distant planets reach opposition, it is very close to seeing them full, but especially with Jupiter and Saturn, full is really the only way we get to see them. As they are further from the Earth than the Sun, no matter when we look at them, the side facing us must be roughly facing the Sun as well. We never get to see them go through phases like Venus and Mercury. This is mostly true for Mars as well. We are still closer to the Sun than Mars, but the difference isn’t as great. We’ll never exactly see a Half Mars, but when Mars halfway in it’s orbit, between being behind the Sun and behind the Earth, we do get to see a chunk of the night time side. It is similar to a Gibbous Moon, not exactly a Full Moon but definitely more illuminated then a Half Moon. For planets closer to the Sun than us, such as Venus and Mercury, this is when we would see them at a half and at their greatest elongation, but for Mars this is halfway between it’s best position and being invisible behind the Sun.

Pushing forward from this point, Mars will get more full looking and closer to us. Eventually it will grow to a Full Mars and be at it’s closest to us once we reach opposition. This will put mars just 96 million kilometers away from us, in January 2025. As we come up to the end of 2024 we will finally be catching up to mars, and in January we will have finally caught up to it. After that of course, we have to start pulling away in front. We are still moving faster than Mars, so from the point of it’s opposition it will start to lag behind the Earth. It will continue to chase us around the Sun, but eventually the Earth will pull so far ahead that it will drop behind the Sun again. The time around the opposition of Mars is when we get to see the retrograde motion of Mars. As we pull ahead of it it will appear to shoot backwards against the background of stars, appearing to change direction. There is a video and corresponding post all about retrograde motion and the apparent motion of Venus and Mercury, it also covers a little bit of the motion of Mars. The important point is that all the planets are moving, the way they appear to behave in our sky is due to the combining influence of our motion and their motion.

Taking a close look at Mars over longer periods like this also lets us see both of Mars’ poles. Mars has an axial tilt just like the Earth, for part of its year the South Pole Faces the Sun and for the other part the North Pole is tilted towards the Sun. This gives Mars a very similar pattern of seasons to the Earth, and the seasons are reversed on opposite hemispheres just like ours. It also means that different poles point at us at different times, so not only can you see different parts of Mars as it turns by going out a few nights in a row, you can see amounts of each hemisphere by going out at different times of the year. With a year about twice as long as the Earth’s, it takes twice as long for the Seasons to change as well. When we take a closer look in the video, we start by seeing the South Pole. As the South Pole is facing us, it must also be roughly facing the Sun, making it summer time in the Southern Hemisphere of Mars. There is an ice cap visible down in the South, but it is quite small. A lot of the carbon dioxide that would be frozen to the South Pole in winter has sublimed, from dry ice back into gas. There is still frozen water at the South Pole, and that sticks around all year long. As Mars continues in its orbit, the South Pole will begin to tilt away from the Sun, and as the temperature lowers the carbon dioxide freezes back down causing the ice cap to grow. By the time the ice cap is at it’s biggest, the South pole is pointed away from us and the Sun making it a bit harder to see. This means that the North Pole will have come into view, pointing towards us as it comes into summer, with its ice cap shrinking away, with carbon dioxide subliming into the atmosphere leaving behind frozen water. The ice caps on Mars are mostly made from dry ice or frozen carbon dioxide, which sublimes directly into the air rather than melting into a liquid, but otherwise this seasonal variation is very similar to what happens with our own ice caps. Many planets have less of an axial tilt, which makes them less seasonal and keeps both hemispheres more similar throughout the year.

Coming back to today, Mars is still just a faint dot above the Morning horizon, but with this look into the future you know what to expect, which should include some nice conjunctions as Jupiter and Saturn overtake Mars later this year. Until then, we’ll keep looking at what’s in the sky and what’s changing, and with the equinox and an eclipse coming up there will be plenty to look at, so I hope you’ll join me back here then.