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Mars Perihelic Opposition

All planets in the Solar System, including the Earth, orbit the Sun more or less on the same plane. The closer a planet is to the Sun, the shorter is its orbital period. When the Earth moves into a position between the Sun and a planet and the three celestial bodies almost align, an observer on Earth will find that the planet and the Sun are in opposite directions. This astronomical phenomenon is called a planetary opposition. When this occurs, the planet comes closest to the Earth, which is the best time for observation by stargazers. In July, 2018, the Earth will orbit into a position between the Sun and Mars. An astronomical phenomenon called a Mars opposition will occur. Since Mars has an elliptical orbit, the distance between the Sun and Mars has a difference of about 42.5 million kilometers between its perihelion and aphelion. When a Mars opposition occurs at a position near Mars' perihelion, it is called a Mars perihelic opposition. When this happens, Mars will be much closer to the Earth and appears much bigger and brighter. The most recent Mars perihelic opposition occurred in 2003 while the next will occur on 27 July 2018.

Mars Perihelic Opposition

When a Mars perihelic opposition occurs on 27 July 2018, the Earth and Mars will be about 57.6 million kilometres apart. The disc of Mars with a diameter of 24 arc seconds (less than 0.007 degree) can be seen with the aid of an astronomical telescope whereas an orange-red star with brightness at a star magnitude of -2.8 can be seen with the naked eye. It will be one of the brightest stars in the night sky at that time. Mars will rise in the East at 7:17 pm on that day. It will reach its highest position on the sky at 0:38 am on the next day, and then set in the West at 5:55 am. The brightness as well as size of Mars will not much vary within a period of 10 days before and after a Mars perihelic opposition. Interested observers may watch it during the period from 17July to 6 August. Please check the information in the table below for reference:

Date Brightness
(Star magnitude)*
Size
(arc second)**
Suggested time for observation
17.7.2018 - 26.7.2018 - 2.6 to - 2.8 23.44 to 24.24 20:00 – 04:00
27.7.2018 - 2.8 24.24
28.7.2018 - 6.8.2018 - 2.8 to - 2.7 24.24 to 24.19

* The lesser the star magnitude of a star, the brighter it is.
** 1 arc second is equal to 1/3600 degree
Source: United States Naval Observatory

Quiz:
Did you know that the rate of change of Mars' brightness and size are different during the 10 days before and 10 days after the opposition? Why?

Mars' brightness and its size depend on its distance from the Earth. When Mars is at the farthest distance from the Earth, it has a star magnitude of about 1.8 and a size of about 3.5 arc seconds only. During the Mars perihelic opposition this year, Mars will be about 70 times brighter and 7 times larger in apparent diameter than when it is at the farthest distance. Using an astronomical telescope, some Martian surface features such as the polar caps can be seen. Mars' rotation can even be identified by long-time observation. A Mars perihelic opposition occurs every 15 or 17 years. It will attract the attention of many astronomy observers.

Coincidentally, a total lunar eclipse will also occur during the period when the Mars perihelic opposition occurs this year. At that time, a reddish moon will appear next to the bright and orange-red Mars. Both celestial bodies will be very close to each other. This would indeed be a spectacular view.

For more information about the lunar eclipse, please click here.

Photo of Mars taken during the Mars perihelic opposition in 2003
Photo of Mars taken during the Mars perihelic opposition in 2003 using an astronomical telescope. The polar cap of Mars can be seen at its top.

two images of Mars were taken by the Hubble Space Telescope during the Mars perihelic opposition in 2003
For space telescopes, a Mars perihelic opposition renders them the best time to observe Mars. The above two images of Mars were taken by the Hubble Space Telescope during the Mars perihelic opposition in 2003.

Rise and Set Times of Mars (For reference)

Date Rise Time
Transit Time
(+1 day)
Set Time
(+1 day)
25.7.2018 19:26 00:48 06:05
26.7.2018 19:21 00:43 06:00
27.7.2018 19:17 00:38 05:55
28.7.2018 19:12 00:33 05:49
29.7.2018 19:07 00:28 05:44

Mars Opposition Q & A

Citizen asks:
How big is Mars during its perihelic opposition? Can it be seen by the naked eye?

Curator answers:
The apparent diameter of Mars during its perihelic opposition this year is about 24 arc seconds. An"arc second" is a very small unit. We divide 1 degree into 60 arc minutes, and 1 arc minute into 60 arc seconds. That means 1 arc second is equal to 1/3600 degree, and 24 arc seconds are less than 0.007 degree! The moon we normally see has an apparent diameter of about 31 arc minutes, which is more than 70 times larger than that of Mars during perihelic opposition! Therefore, observing Mars with the naked eye, we can only see a bright orange-red star. A professional observer using a telescope with more than 100x magnification should be able to watch some features on the Martian surface.

Mars Moon Comparison
Even during a perihelic opposition, the apparent size of Mars is negligible when compared with that of the Moon. 

Citizen asks:
Why does a Mars opposition occur about a bit more than every two years, and a Mars perihelic opposition occur every 15 or 17 years ?

Curator answers:
The Earth takes 365.2 days for one orbit while Mars takes about 687 days. The following formula can be used to calculate how long it takes for the two planets to show the same positional configuration (also called Synodic Period):

Synodic Period

Note: PS is the synodic period, PE is the orbital period of Earth, PM is the orbital period of Mars.

Through simple calculation, we found that the synodic period between Earth and Mars is 780 days, or 2 years and 50 days. A Mars perihelic opposition occurs only when Mars is near its perihelion, and there is only one fixed perihelion in the orbit of a planet. A Mars opposition occurs every 780 days, but the orbital period of Mars is 687 days only. That means it takes 93 extra days for Mars to orbit after one revolution before Mars opposition occurs again. Hence the number of days between a Mars opposition and a Mars at perihelion is increased by about 93 days annually. There are at least 687÷93=7.4, or 7 to 8 Mars oppositions between two perihelic oppositions, i.e. about 15 or 17 years.