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2018: Mars Perihelic Opposition


Figure 1: With opposition on July 27, Mars completes a perihelic opposition on July 27, 2018.

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After its solar conjunction in late July 2017, Mars begins a 768-day cycle during which it reaches opposition; that is, Earth is between the sun and the Red Planet.  The opposition occurs July 27, 2018.

This one is special as it occurs when Mars is near its closest point to the sun: perihelion.  And so this opposition is known as a “perihelic opposition.”  Such oppositions occur every 15 years or 17 years. Oppositions occur about every 25 months, but the ones when Mars is near perihelion are of special interest.

This article explains the entire apparition of Mars.

Here we describe part of the appearance, near opposition.  Mars orbit is not a perfect circle; it’s an ellipse.  If Earth passes Mars when it is near perihelion, terrestrial telescopes capture exquisite pictures of this planet.  Before the 2018 opposition, the 2003 opposition occurred when Mars was very close to perihelion.  Other very close perihelic oppositions occurred:

Figure 2:NASA Image from the 2003                       perihelic opposition

Phobos and Deimos, the moons of Mars, were first observed by Asaph Hall near the time of the perihelic opposition in 1877.  At opposition the planet is in the sky all night as it rises in the east at sunset and sets in the west and sunrise.  It is near its closest to Earth and is at its best observing.

With robot spacecraft roving on the martian surface and revolving above the planet, perihelic oppositions are not as exciting as those in recent history; yet, to watch the planet grow in brightness and appear as a small ocher orb in a telescope is an exciting opportunity.

Events Around Opposition

Because Mars is near our planet, it moves around the sun at about half the speed of Earth.  Our planet catches and moves past Mars in a little over two years.  So compared to most other observations of the planets, the intervals between oppositions seem long.

Figure 3:   Orbital Chart. This chart shows the overhead view of Earth and Mars during 2018

The two charts (Figure 1 and Figure 3) show two different views of the same events.  The former chart shows the view of what we see from our viewpoint on Earth.  The latter shows the orbital paths of the two planets as viewed from above the solar system.  (Click the charts to see them larger.)

Here are the events as noted on the charts:

One of the biggest challenges of our ancestors was to explain the retrograde motion of Mars (Figure 1), Jupiter, and Saturn.  From a sun-centered explanation of the solar system, these outer planets seem to stop and backup as a faster moving Earth catches and passes them.  From Figure 3, note that the planets do not stop at any time in their orbital motions.

Date Distance
(Million Miles, rounded)

Brightness (m)

Brightness Change
(From Previous Date)
April 27, 2018        81        -0.3
June 26, 2018        44        -2.08 5.2 x brighter
July 27, 2018        35.8        -2.78 1.9 x brighter
July 31, 2018        35.7        -2.78 No Change
August 27, 2018        41        -2.2 2 x dimmer
September 15, 2018        47        -1.7 1.6 x dimmer
October 13, 2018        62        -1.0 1.3 x dimmer

Table 1:  Some properties of Mars, 2018

The table above summarizes the preceding text .  The magnitude column shows the brightness of Mars on a numerical scale.  Smaller numbers indicate that the planet is brighter.  The scale becomes more negative numerically for brighter objects.  (The sun’s magnitude is -26.5.  After all it is so bright that it causes daytime.)  From April 27 through July 27, Mars increases in brightness 2.48 magnitudes.  To the eye that corresponds to a 10 times increase in brightness. Each step on the magnitude scale is about a 2.5 times change in brightness.  In comparison here are magnitudes of some other bright stars:

An exciting viewing opportunity occurs during the next year.  With this opposition occurring in July 2018, it is easily observed.  The next martian opposition is October 13, 2020 with the next perihelic opposition September 15, 2035 at a distance of 35.4 million miles from us.  Happy observing!

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