2018: Mars Perihelic Opposition

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:

  • August 18, 1845
  • August 23, 1924
  • August 10, 1971
    See this source for a listing of other perihelic oppositions.

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 it 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.

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 about the solar system.  (Click the charts to see them larger.)

Here are the events as noted on the charts:

  • April 27, 2018:  Mars is over 81 million miles from Earth.  It is brighter than any other “star” in the region, except for Jupiter which is 60 degrees to the west of Mars.  Mars rises in the southeast at about 1:30 a.m.
  • Between these two dates, Mars appears to move eastward compared to the starry background.  In June Mars appears to slow down its eastward motion.
  • June 26, 2018:  Mars stops moving eastward and begins to slowly move westward compared to the starry background.  It is 44 million miles away and has grown five times in brightness. The planet rises at about 11 p.m. It is important to note the Mars appears as a star and during this opposition, it cannot be seen as a planetary globe with the unassisted eye.  A telescope is needed  to see its shape.
  • July 27, 2018:  Mars is at opposition.  In one month it nearly doubles in brightness.  It is nearly as bright as Jupiter, yet appears starlike without a telescope.  It is 35.8 million miles away.  It rises in the southeast at sunset and sets in the southwest at sunrises.  It is highest in the south around midnight.  This is the best time to view the planet through a telescope.
  • July 31, 2018: Mars is closest to Earth (closest approach).  It is 35.7 million miles away.  Mars is not necessarily closest at opposition because of its elliptical orbit.  Mars is still moving toward perihelion gradually getting closer to the sun each day.
  • August 27, 2018:  Mars stops retrograding.  It is 41 million miles away.  It is still bright but it is noticeably diminished.  Earth is now pulling away from Mars.  Mars begins moving eastward again compared to the starry background
  • September 15, 2018:  Mars is a its solar perihelion, 47 million miles away from Earth.
  • October 13, 2018:  (The last date charted.)  Mars is 62 million miles away from Earth and distinctly dimmer than it was at opposition.  As Earth moves away from Mars, the Red Planet appears to pick up speed as moves eastward.

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)

Magnitude
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 negative 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:

  • Sirius, -1.44 (the brightest star in the night sky)
  • Arcturus, -0.05
  • Betelgeuse, 0.45
  • Aldebaran, 0.87

An exciting viewing opportunity occurs during the next year.  With this observation 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!