Observing the planets and their apparent relative sizes

When observing the planets in astronomy it can be quite surprising to see the different sizes that appear in the eyepiece and how this can change over time.

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This winter there are a lot of planets around (‘this summer’ in the Northern Hemisphere). Early in the evening you’ll easily see the very bright Venus and when the Sun has got far enough below the horizon you’ll start to see Jupiter high to the North West. By mid evening Saturn becomes visible and a few hours later, along comes Mars. What’s so special about all of the planets visible this time is that we also get to have a closer look at Mars due to it being in opposition. This is where Earth sits directly between Mars and the Sun making the distance between Mars and the Earth as small as it gets, depending on where Mars is in its elliptical orbit of the Sun. Because Jupiter and Saturn are such a long way away, our position in the orbit doesn’t make a lot of different to what we see, there is an appreciable difference it’s just not as striking as when Mars gets to opposition. Of course with all of these planets visible we want to make the most of it so it’s worth thinking about just how now is a great time to observe them.We wondered what the comparitive sizes were between the planets if you imagined that they are observed through the same telescope and eyepiece at different parts of their orbit.

Saturn is about 9.06 Astronomical Units (AU) from the Earth (an astronomical unit is about 150,000,000 kilometres or the distance from the Sun to the Earth). Saturn can get as far a way as 11 AU when we are on the opposite side of the Sun to Saturn, obviously we can’t see Saturn then. It only ever gets a bit closer than it is now, as we are quite close to opposition with the giant ringed planet. At the moment Saturn is 18.3 arcseconds in size (this is the planet disk and doesn’t include the rings). When it’s on the opposite side of the Sun from us then it is considerably smaller at 15.1 arcseconds, about 82% of the largest size we get to see Saturn. So if you want to capture the Cassini Division in Saturn’s rings then now is the time to do it because by January it might be too far away (and, of course, the sun will be in the way!).

Saturn at opposition on the left and conjunction on the right (obviously not a real photo!)

Jupiter is 4.62 AU from us at the moment, which is also a very good position, back in May we were near at opposition with Jupiter which put us a bit closer at 4.4 AU. What this means is that back in May Jupiter appeared 44.8 arcseconds in size and now it’s slightly smaller at 42.7 arcseconds, about 5% smaller in apparent diameter. That’s not much different but enough to be noticeable and Jupiter is getting further away each day as we advance in our rotation aroun the Sun. The farthest we get from Jupiter is when where on the other side of the sun at 6.35 AU, at this time Jupiter’s apparently size is about 31 arcseconds (about 70% of the size when Jupiter is at opposition). This is another reason to get out and see Jupiter now! Because it’s only going to get smaller, until next year.

Jupiter at the two extremes, notice more of a difference than with Saturn

Now Mars is where the distance really matters and can make a huge difference to what we can see and photograph on the red planet from our sunny vantage point on Earth. When Mars is at opposition it will be 57.7 million kilometres away and 24.3 arcseconds in size (still heaps smaller than Jupiter, just shows how massive Jupiter is!!). At the moment Mars is 75.3 million kilometres away and 18.6 arcseconds in size so in only six weeks it will be 17.6 million kilometres closer to us and nearly 30% bigger in diameter. As a comparison, when we are on the opposite side of the Sun to Mars, like in July last year, we were 396 million kilometres away and Mars was only 3.5 arcseconds in size, just 14% of the size we will observe at the end of July. What does this tell us? Well, the best time to see Mars for a couple of years is going to be in the two to three months, so make the most of this amazing spectacle and hopefully the dust storm that’s raging on the red planet at the moment subsides so we can see some features when it gets to opposition.

Mars at its closest and farthest away, a huge difference!

The featured image shows how all of the planets compare in their large sizes to the left and there smallest to the right. The illustrations are representative of the comparative apparent sizes – not suitable for scientific measurements!