Monday July 4th is the autumnal equinox on the planet Mars as the planet transitions from summer during its 684 Earth day orbit around the Sun.
Seasons on Mars are marked by the planet’s heliocentric longitude coordinates using the position of Mars along its orbit around the Sun. At the Martian spring equinox Mars is at 0o longitude.
Each seasonal start/ending point is 90 degrees apart, but because of an elliptical-shaped orbit each Martian season is of varying lengths. Mars is at its greatest distance from the Sun, aphelion, before it reaches the Martian summer solstice when Mars is at 70o longitude. Perihelion, its closest to the Sun, is when Mars is at 250o longitude.
Eccentricity of Mars and Earth for comparison.
Mars: 0.0934 – Earth: 0.0167
I’m not exactly sure why this particular date is used but by international agreement astronomers have selected 11 April, 1955 as 0 degrees for year 1 of this Martian calendar. What this means is that Monday July 4th at 16 UT (11 am CDT) Earth time, is the start of autumn for Martian year 33 using the Earth-designed Martian calendar system.
Mars is currently visible in the evening skies over the southern horizon after sunset local time.
0 degrees — Spring Equinox — Jun 18 2015
90 degrees — Summer solstice — Jan 03 2016
180 degrees — Fall Equinox — Jul 04 2016
270 degrees — Winter Solstice — Nov 28 2016
0 degrees — Spring Equinox — May 05 2017
90 degrees — Summer solstice — Nov 20 2017
180 degrees — Fall Equinox — May 22 2018
270 degrees — Winter Solstice — Oct 16 2018
Learn a little (or a lot) more about Mars at the NASA/JPL Mars Curiosity mission web site.
Here is approximately 3 minutes worth of Mars from the Orbit performance.