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## The 2016 opposition of Mars

On Sunday 22 May, Mars will be at opposition. This is the time when Earth passes Mars in their respective orbits around the Sun, and so Mars is at its closest to us. Because it is at its closest to us it is also at its brightest; an opposition of Mars usually presents the best time to see the planet. Earth, of course, takes 365.25 days to orbit the Sun. Mars takes 687 days, this is known as is its sidereal period. The time between each opposition is 779.96 days, which translates to 2 years and 49.5 days, or just under 2 years and 2 months. This is known as the synodic period for Mars.

For this particular opposition, Mars is between the constellations Libra and Scorpio. Below is a diagram showing where Mars will be at 23:00 BST (11pm) on the night of Sunday 22 May as seen from Cardiff. Unfortunately, for places as far north as Cardiff (which is at a latitude of 51.5 degrees north), this part of the sky never gets particularly high above the horizon. In this diagram, at 23:00, Mars only has an altitude of 11 degrees above the horizon, which is quite low. The highest Mars will appear in the sky during this opposition is at around 1am, then it will  be about 17 degrees above the horizon. This is still quite low but, thankfully due to Mars’ brightness, it should be easily visible.

The position of Mars at 23:00 on Sunday 22 May as seen from Cardiff. Mars is in the constellation Scorpio, at about 1 o’clock from Antares if you imagine a clock face. At about 8 o’clock from Mars and about 10 o’clock from Antares is Saturn. On Sunday, the Moon will also be nearby, and as it is near full it may impede viewing Saturn.

Although the opposition happens on Sunday 22 May, do not worry if you are not able to see Mars that night (due to cloud or some other reason). Mars will remain bright in the sky for the next several weeks. In fact, if you have been looking at Mars over the last few months you will have noticed that it has brightened considerably as it has approached opposition. Of all the planets, Mars is the one which shows the biggest change as the Earth approaches it, far more than the others.

For example, in late January Mars was nearly 20 times fainter than  it will be on 22 May. Even in late March it was nearly 6 times fainter, now it has brightened to outshine everything else in that part of the sky. But, it will still be pretty bright for the rest of May and into June. If you are lucky enough to be going away anytime between now and mid-June to more southerly latitudes, then Mars will appear much higher in the sky. People in the Southern Hemisphere are getting a much better view of this opposition than those of is in northern latitudes.

Because Mars will be so bright, it should be very easy to find. It has a distinctive red appearance, but not quite as red as Antares, the red giant star in Scorpio. You can use Mars to find other objects nearby. Imagining a clock face, Antares is at about 8 o’clock from Mars, and about 20 times fainter. At about 9 o’clock from Mars and 10 o’clock from Antares is Saturn, which is currently about 8 times fainter than Mars. On the night of Sunday 22 May, the Moon will be quite close to Saturn, and it will also be nearly full. Although this shouldn’t affect your being able to see either Mars or Saturn, you may find that you get a better view of both if you look a few days before or later, when the Moon won’t be nearby.

By late June Mars will have faded to being about 2 times fainter than it will be at opposition, but still a lot brighter than it was in March or April. By July it will have faded further, being about 3 times fainter than when at opposition, and about the same as it was in late April. But, if you are waiting for your summer holidays to get to southern latitudes, Mars will be easily visible in late July and August, and will still be brighter than either Antares or Saturn.

## Do oppositions vary?

But, not all oppositions are equal. The planets do not orbit the Sun in circular orbits, but rather in elliptical orbits. We measure how far from being circular an ellipse is by something called an ellipse’s eccentricity; an eccentricity $e=0$ means that the ellipse is a perfect circle (a circle is just a special case of an ellipse), an eccentricity greater than this means it deviates from a circle. The eccentricity of the Earth’s orbit is $e=0.017$, pretty close to being a circle. However, Mars has a more elliptical orbit, the eccentricity of its orbit is $e=0.09$. The only planet with a more eccentric orbit is Mercury. The eccentricity of Mars’ orbit means that the distance between Earth and Mars when they are at opposition varies. For this particular 2016 opposition the distance will be 76.3 million kilometres (about 47 million miles). The figure below shows the distance between Earth and Mars for all the oppositions between 2001 and this one in 2016.

Oppositions of Mars from 2001 to this current 2016 opposition. The figure shows the dates of these oppositions, and also the distance in millions of miles. For this 2016 opposition, the distance will be about 47 million miles, or about 76 million km

During the August 2003 opposition, Mars got to within 55.8 million kilometres. This was, in fact, the closest opposition for many thousands of years; it has been calculated that the last time Earth got this close to Mars was in the year 57,617 BC! The next opposition which will be as close as the 2003 opposition will be in 2287, quite a wait.

## Oppositions from 2012 to 2027

Below is a table of the dates of oppositions of Mars from 2012 to 2027, showing for each opposition the constellation in which Mars appears, its distance from Earth (in both Astronomical Units and millions of kilometres), and its apparent magnitude and apparent angular size (in arc seconds). Remember, apparent magnitude is a negative system, a lower number means brighter. As the figure above shows, the closest opposition that has occurred recently was in August 2003. Close oppositions happen when Mars is near its perihelion (closest to the Sun), because this will also mean that it is closer to Earth. These are called perihelic oppositions, and the time between perihilic oppositions is either 15 or 17 years (it alternates).

As the table below shows, the opposition in July 2018 will actually be better than this 2016 opposition, with Mars getting to within 57.5 million kilometres and brightening to a magnitude of -2.8 (which is twice as bright as during this opposition). However, it will be in the constellation Capricorn, which is even further south in the sky than Scorpio; so even lower in the sky for those of us living in northern latitudes.

Oppositions of Mars from 2012 to 2027

Opposition Date

Constellation

Distance (AUs)

Distance (million km)

Apparent magnitude

Apparent size

(arcseconds)

2012 March 3

Leo

0.6745

100.5

-1.2

13.9

2014 April 8

Virgo

0.6219

92.7

-1.5

15.1

2016 May 22

Scorpio

0.5101

76.3

-2.0

18.4

2018 July 27

Capricorn

0.3862

57.5

-2.8

24.2

2020 October 13

Pisces

0.4181

62.3

-2.6

22.4

2022 December 8

Taurus

0.5492

81.8

-1.8

17.0

2025 January 16

Gemini

0.6435

95.9

-1.4

14.5

2027 February 19

Leo

0.6780

101.0

-1.2

13.8

## How has Mars brightened over the last year and how will it fade over the coming months?

For those of you who have been looking at Mars over the last few months, you will have noticed how much it has brightened. Here is a table showing the apparent magnitude of Mars on the 21st of each month. If you do not manage to see it during May do not worry, it will remain fairly bright well into June and July and, even in August, it will be outshining Antares and Saturn. If you are heading further south during the summer months, then it would be an ideal time to see Mars.

The distance, apparent magnitude and size of Mars from June 2015 to December 2016

Date

Constellation

Distance

(AUs)

Apparent

Magnitude

Apparent Size

(arcseconds)

2015 June 21

Taurus

2.58

+1.5

3.6

2015 July 21

Gemini

2.58

+1.7

3.6

2015 August 21

Cancer

2.54

+1.8

3.7

2015 September 21

Leo

2.43

+1.8

3.9

2015 October 21

Leo

2.27

+1.7

4.1

2015 November 21

Virgo

2.04

+1.6

4.6

2015 December 21

Virgo

1.78

+1.4

5.3

2016 January 21

Libra

1.48

+1.0

6.3

2016 February 21

Libra

1.16

+0.4

8.1

2016 March 21

Scorpio

0.88

-0.2

10.7

2016 April 21

Scorpio

0.63

-1.2

14.8

2016 May 21

Scorpio

0.51

-2.1

18.4

2016 June 21

Libra

0.54

-1.6

17.3

2016 July 21

Libra

0.67

-1.0

14.0

2016 August 21

Scorpio

0.84

-0.4

11.2

2016 September 21

Scorpio

1.01

0.0

9.2

2016 October 21

Sagittarius

1.19

+0.3

7.9

2016 November 21

Capricorn

1.38

+0.6

6/8

2016 December 21

Aquarius

1.57

+0.8

6.0

Hopefully you will get a chance to see Mars over the next few weeks or months. If you get a chance to look through a small telescope or binoculars you may be able to make out white near the poles of Mars, these are polar ice caps and are mainly frozen water, with some frozen carbon dioxide. The frozen carbon dioxide has been measured to sublimate into the atmosphere when that particular polar region is in summer, but the water remains permanently frozen. When the pole loses its sunlight, some of the carbon dioxide in the atmosphere freezes again, forming part of the polar cap.

Mars during the January 2010 opposition. Image credit Alan Friedman.

Here is a picture I took of Mars over Cardiff City Hall on Sunday night/Monday morning (15-16 May), at 01:30. For details of the exposure time and aperture etc., see the caption.

Mars over Cardiff City Hall on Sunday night/Monday morning (15-16 May 2016). Mars is just to the right of the clock tower. To the left at about the same height is Saturn, and to the left and low down is Antares. This picture was taken at 01:30 and is a 10s exposure with f/4.4, 44mm focal length and ISO 100 using a Nikon D3200 DSLR.