THE OPPOSITION OF MARS - 2024-2025

A painting of the Martian North Polar Cap (NPC) showing the polar layered deposits (PLD) as a record of the changes in the Martian climate over a period of approximately 120,000 years. Painting by Carlos E. Hernandez. 

Amateur astronomers will enjoy the new year in 2025 by observing an opposition of the red planet Mars. An opposition of the planet Mars occurs approximately every 26 months (2 years and 2 months or synodic period of 789 days). Mars will shine as a “brilliant red star” in the constellation of Gemini the Twins at opposition. 

The Red Planet Mars

The planet Mars orbits the sun at an average distance of approximately 142 million miles (or 228 million kilometers). This distance is approximately 1.52 times the distance of the earth from the sun, also known as an astronomical unit (AU: one astronomical unit is equal to approximately 93 million miles or 150 million kilometers). Mars orbits the sun in a period of 687 days (1.88 years or 1 year and 11 months), also known as the sidereal period. The synodic period (time required for a celestial object to return to the same position in the sky in relation to another celestial body) for the planet Mars is 780 days (approximately 2.2 years or two years and two months), at which time the planet is at opposition. The opposition of a superior planet (a planet that orbits the sun outside the earth’s orbit) occurs when it is directly opposite (180 degrees) the sun as visible from the earth. The planet earth and Mars are at their closest distance during an opposition.

The Martian seasons are longer than the Earths (365.25 days or 1 Earth year) as it is an outer planet and orbits the Sun in a period of 687 days (1.88 years). The seasons on Mars are irregular in length and the longest season is Northern Martian Spring (194 Sols; 1 Sol equals 24 hours 37 minutes 22.7 seconds) and the shortest season is Northern Martian Autumn (143 Sols). In describing the Martian seasons planetary scientists use the term Ls (or Longitude of the Sun along Mars’ ecliptic or orbital plane) in which

Ls 0° = Northern Martian Spring, Ls 90° = Northern Martian Summer, Ls 180° = Northern Martian Autumn, and Ls 270° = Northern Martian Winter (opposite seasons for the Southern Hemisphere).  The current Martian season is ~85° behind the season on Earth (e.g., Northern Spring on Earth equals Northern Martian Winter).

The planet Mars is the 4th planet from the sun. Mars orbits the sun at an average distance of 146 million miles 235,000,000 kilometers, 1.52 astronomical units (AU, one astronomical unit equals approximately 93 million miles or 150 million kilometers).

The diameter of Mars is equal to 4221 miles (6792 kilometers, or 53% the Earth's diameter). The rotation period of Mars is equal to 24 hours 37 minutes 22.7 seconds (or 1.03 days, also known as a Sol). The axial tilt of Mars is 25.2 degrees compared to the Earth's axial tilt of 23.5 degrees. The mean surface pressure on the planet Mars is 600 pascals (Pa, or 0.087 PSI). This ranges from 30 pascals (0.0044 PSI) and the Olympus Mons to 1155 pascals (0.1675 PSI in Hellas Planitia).

The planet Earth in comparison to the planet Mars. Mars is approximately 53% the diameter of the Earth and 10% of its mass. One Martian day (or Sol) is 24 hours 37 minutes 22 seconds. Surface gravity upon the surface of Mars is 38% that of the Earth’s.

The Moons of Mars

Mars has two satellites named Phobos (“Fear” in Greek) is the inner potato-shaped moon (mean radius of 7 miles (11 km)) of Mars that orbits the red planet at an average distance of 3,700 miles (6,000 km) from the Martian surface. It orbits Mars in a period of 7 hours 39 minutes and appears to rise in the west and move across the Martian sky in 4 hours 15 minutes or less, and set in the east, twice each Martian day. 

Deimos (“Terror” in Greek) is the outer moon (mean radius of 3.9 miles (6.2 km)) that orbits Mars at 14,580 miles (23,460 km) in a period of 30.3 hours rising in the east and setting in the west. Since its period is longer than the Martian day (24.7 hours (1 Sol)) an observer on the Martian equator would see it rise and set after ~2.5 days.

Both Martian moons were discovered by the American astronomer Asaph Hall, III (1829-1907) at the United States Naval Observatory in Washington, D.C. in 1877 (August 12 for Deimos and August 18 for Phobos).

Mars Oppositions (Aphelic and Perihelic)

Mars has a 15.8-year periodic opposition cycle consisting of three or four aphelic oppositions and three consecutive perihelic oppositions. The synodic period of Mars is approximately 780 days (~2 years and 2 months) between oppositions of Mars. Perihelic (within 60° of perihelion (Ls 250°)) oppositions of the planet Mars may approach the Earth to within 34.7 million miles (55.8 million km) and sustain a maximum apparent diameter of 25.11 arc-seconds. The southern hemisphere of Mars is typically visible from the Earth during perihelic oppositions. Aphelic oppositions (within 60° of aphelion (Ls 70°) are typically over 62 million miles (100 million km) and the northern hemisphere of Mars is usually visible.

A simulation of the apparent size of the planet Mars over a period between August 6, 2024 to May 15, 2025. Mars images courtesy of Stellarium.

Mars at Opposition

The planet Mars will be located in the constellation of Ophiuchus the Serpent Bearer in August, 2024 then move among the stars over Sagittarius the Archer, Capricorn the Goat, Aquarius the Water Bearer, Pisces the Fish, Aries the Ram, Taurus the Bull, and finally Gemini the Twins at opposition on January 16, 2025.

Mars will be visible above the celestial equator until August 29, 2025. The retrograde motion (the outer planets normally follow the rotation of the Earth in a western to eastern direction among the stars. As the Earth (inner planet) catches up to the Mars (outer planet) that is orbiting the Sun at a slower velocity then the outer planet appears to go “backwards” among the stars an is now moving in an eastern to western direction. The outer planet may appear to form a loop at this time.) of Mars begins on December 6, 2024 (11.6° Ls) and continue until February 24, 2025 (48.4° Ls) where it will appear to move westward over Taurus the Bull.

The current opposition of Mars is considered to be an aphelic one as the orbital longitude of Mars will be 39.1° from the aphelion longitude of 70° Ls. The closest approach of the planet Mars to the Earth does not always occur on the same date as that of opposition due to the orbital eccentricity of the two planets. The closest approach of Mars to the Earth for the current apparition occurs on January 12, 2025 (1338 UT, 29.2° Ls) at a distance of 0.642825 astronomical units (AU; The average distance of the Earth from the Sun. One astronomical unit equals approximately 93 million miles or 150 million km) or 59,703,891 miles or 90,084,099 km. Mars will then attain an apparent diameter of 14.6 arc-seconds. It will appear slightly smaller in apparent diameter at 14.5 arc-seconds on the date of opposition of January 16, 2025 (0232 UT, 30.9° Ls). Mars will be visible in the sky for approximately 10 months after opposition until approximately November 24, 2025 when it be lost in the glare of the Sun.

North Polar Cap of Mars

Although the planet will be further away from the Earth during the current opposition it will afford amateur astronomers an opportunity to have a better look at the North Polar Cap (NPC) and the surrounding regions. The North Polar Cap (NPC) of Mars is typically visible during aphelic oppositions of Mars whereas the South Polar Cap (SPC) is more prominent during perihelic oppositions. The growth and recession the North Polar Cap (NPC; as well as the South Polar Cap (SPC)) is due to the axial tilt of the planet Mars being similar to that of the Earths (25.19° versus 23.44°). The spring thaw of one polar cap leads to the formation of the opposite cap.

The North Polar Cap (NPC) is composed of four layers. The bottom layer is called the basil unit (or foundation of the NPC) and is composed of sand and dust cemented by water ice. This layer is estimated to be approximately 0.6 miles (3,300 feet or 1,000 meters) in thickness. The layer above the basal unit that forms the majority of the NPC is called the polar layered deposits which measure up to 1.2 miles (6,600 feet or 2,000 meters) in thickness. The polar layered deposits are formed by a mixture of atmospheric dust and the polar layered deposits are therefore a reflection of changes in the Martian climate over time, similar to tree rings.

The axial tilt of the planet Mars is believed to have vary between 15 to 50 degrees over a 120,000 year period with a longer variation period of 1.2 million years. It is a coincidence that the axial tilt of Mars is approximately 25.2 degrees at this time, compared to to the Earth’s axial tilt of 23.5 degrees. Each layer of dust and ice may represent a period of 8,000 to 8 million years, but the average is believed to be 120,000 years based upon cyclic changes in the Martian orbit.

The NPC has a permanent residual water ice cap that is composed of water ice (H2O) that sits above the polar layered deposits with a temporary thin seasonal ice cap, or veneer, (approximately 3 feet (1 meter) thick) of frozen carbon dioxide (CO2, or dry ice). The South Polar Cap (SPC) has a permanent frozen carbon dioxide cover that is approximately 26 feet (8 meters) thick. The NPC can measure up to about 750 miles (1,200 kilometers) across, with a maximum thickness of 1.8 miles (3 kilometers). The cap is cut by canyons and troughs that plunge to as deep as 0.6 miles (1 kilometer) beneath the surface, or reaching the basal unit, as measured by the NASA Mars Orbiter Laser Altimeter (MOLA).

The NPC contains approximately 1.6 million cubic kilometers of water ice (H2O; compared to the Greenland ice sheet of 2.85 million cubic kilometers). Spiral troughs are visible over the NPC produced by katabatic winds (produced by high-density air flows from a higher elevation down a slope under the force of gravity).

North Polar Cap Remnants (Outliers)

As the North Polar Cap (NPC) recedes from its maximum Winter extent during late northern hemisphere spring and early summer then three bright projections, or outliers, will become visible to the observer/imager. According to center-of-area Areographic longitude positions these are Ierne 137°, Lemur (Olympia) 200°, and Cecropia 297°. These NPC remnants may be difficult to detect in apertures smaller than 8 to 10 inches (20-25 cm.) and typically require steady seeing conditions.

Rima Tenuis

The Rima Tenuis is a dark rift in the North Polar Cap (NPC) that was discovered in 1888 by the Italian astronomer Giovanni Virginio Schiaparelli (1835-1910) using an 8.6-inch (21.8 cm.) f/14.7 refractor at the Brera Observatory in Milan, Italy. It was confirmed by contemporary astronomers at the time, including Belgian astronomer Francois J. Terry (1846-1911) and French astronomer Henri Joseph Anastase Perrotin (1845-1910; using the famous Nice Observatory 30-inch (76-cm.) refractor).

The Rima Tenuis was not observed again until 1933 and again in 1950. It was again detected by Mars observer Daniel M. Troiani in 1979 as a dark notch in the NPC at 335° West longitude. The Rima Tenuis was detected and imaged by amateur astronomers in 1980 and 1982. On February 22, 1980 Lowell Observatory astronomer Charles F. Capen, Jr. (1926-1986) and the famous British astronomer/author/television host Sir Patrick Alfred Caldwell-Moore (1923-2012) using the Lowell Observatory 24-inch (61-cm) f/16 refractor on Mars Hill in Flagstaff, Arizona USA were able to detect the Rima Tenuis during an observing session.

The mean Martian longitude of the Rima Tenuis is between 129° to 331° West longitude. Unfortunately the Hubble Space Telescope (HST) and orbiting Mars spacecraft have not been able to image and confirm the Rima Tenuis to this date. Dust storms develop over the North Polar Region (NPR) and deposit dust over the NPC and sometimes in the form of dark dust streaks. The Rima Tenuis may possibly be a form of recurrent dust streak over the North Polar Cap (NPC) but it is strange that it forms at the exact longitude on multiple occasions? Future observations and especially images of the North Polar Cap (NPC) during the current aphelic opposition and future oppositions in 2027 (February 19) and 2029 (March 25) may help answer this mysterious feature?

Atmospheric Turbulence (Seeing)

Astronomers observing and imaging upon the Earth observe and image the planets and stars under an atmosphere that affects the quality (sharpness) of the image. Whereas the observer/imager cannot control the steadiness or turbulence of the atmosphere overhead they may take measures to improve the quality of the observation in order to produce the sharpest and steadiest image possible. One of the biggest mistakes that amateur astronomers make is not allowing sufficient cooling time for their telescopes, especially if stored inside a structure (concrete or steel). The observer/imager will declare the night to have turbulent atmospheric (seeing) conditions, but instead the fault lies in heat currents affecting the final image in the instrument. Attempt to leave the instrument open to air for at least one to two hours before a session. The amateur astronomer should attempt to avoid observing or imaging over a hot surface such as concrete, asphalt, or any other surface that might retain the heat of the day. Observing downwind, especially if using a Newtonian reflector, will also improve the image in the eyepiece as the observer avoids body heat flowing across the aperture of the instrument that will produce a turbulent image. The sharpest views of celestial objects are typically obtained as the object approaches the meridian (midline in the sky dividing the east and west horizons) and zenith (highest overhead position of the object). Atmospheric turbulence (or seeing) scales have been developed in order to estimate the steadiness of an object in a telescopic field of view.

Pickering Seeing Scale (0-10; developed by Harvard College Observatory astronomer William Henry Pickering (1858-1938):

1 — Star image is usually about twice the diameter of the third diffraction ring if the ring could be seen; star image 13 arc-seconds (13") in diameter.

2 — Image occasionally twice the diameter of the third ring (13").

3 — Image about the same diameter as the third ring (6.7"), and brighter at the center.

4 — The central Airy diffraction disk often visible; arcs of diffraction rings sometimes seen on brighter stars.

5 — Airy disk always visible; arcs frequently seen on brighter stars.

6 — Airy disk always visible; short arcs constantly seen.

7 — Disk sometimes sharply defined; diffraction rings seen as long arcs or complete circles.

8 — Disk always sharply defined; rings seen as long arcs or complete circles, but always in motion.

9 — The inner diffraction ring is stationary. Outer rings momentarily stationary.

10 — The complete diffraction pattern is stationary.

On this scale 1 to 3 is considered very bad, 4 to 5 poor, 6 to 7 good, and 8 to 10 excellent.

Antoniadi Seeing Scale (I-V; developed by the Greek-French astronomer Eugene Michel Antoniadi (1870-1944):

• I - Perfect seeing without a quiver.
• II - Slight undulations, moments of calm lasting several seconds.
• III - Moderate seeing with larger air tremors.
• IV - Poor seeing; constant troublesome undulations.
• V - Very bad seeing; even a rough sketch impossible.

 Astronomical Equipment

 The telescope is the astronomers tool that captures light and focuses it into an image at the eyepiece field. Whereas the astronomer is at the mercy of the overhead atmosphere during an observing or imaging session, the telescope used by the observer/imager may be optimized in order to take advantage of a steady seeing session. An observer or imager will desire to use the largest aperture instrument (lens or mirror) available in order to increase the contrast and resolution of the object. Apertures larger than 4 inches (10.2 cm.) and especially larger than 6 to 8 inches (15.2-20.3 cm.) will provide the observer/imager a sharp image with maximum contrast if the atmospheric (and sufficiently cooled instrument) allows. This also assumes the amateur astronomer is using diffraction-limited optics (surface accuracy of the lens or mirror surface with a deviation (peak-to-valley) of no greater than 1/4 wave of light).

The instrument, especially a reflector design, must be collimated in order to produce the sharpest image possible in the eyepiece. Several methods of collimating an instrument are available on the internet as well as collimating devices. The eyepiece used is also critical in the formation of the sharpest image possible with the highest contrast in the field of view. Many excellent designs are available to the amateur astronomer including the classic Abbe Orthoscopic, Plossl, Brandon, and newer designs using computer-optimized optics, excellent glass, and even waterproofing of the optics. Typical apparent eyepiece field of views include 45 degrees for the Abbe Orthoscopic to approximately 100 degrees for the modern wide-field eyepiece designs. Planetary observation does not typically require apparent field of views greater than 65-70 degrees but the author has used eyepiece designs between 80 to 100 degrees with excellent results.

In general, an observer should use the highest magnification that will produce the sharpest image, this is assuming diffraction-limited optics (objective and eyepiece), seeing conditions, collimation, and thermal distortion due to stored heat by the telescope. If the atmosphere is very steady an observer may employ a magnification of 60 times the telescopes aperture (inches), although on average 30-40x aperture is more useful in producing a sharp image on average seeing nights. A stepwise increase in magnification in order to produce the sharpest image is a good method of determining the proper magnification for a given observing session. In my experience, an exit pupil of 0.8-1 mm is useful on the majority of nights that an observer will decide to observe objects such as the Moon, planets, and small deep sky objects (e.g. planetary nebula).

Choosing Eyepieces by Scott W, Roberts

Planetary Filters

The amateur astronomer has a very useful accessory in the use if color filters in the observation and imaging of the Moon and planets. Color filters are sold by a variety of manufacturers and are available in 1.25-inch (32 mm) and 2-inch (50 mm) sizes that may be screwed unto the bottom of properly fitted eyepieces as well as CCD cameras. Color filters transmit a specific wavelength of light based upon their respective color. 

Drawing Mars at the Telescope

Making drawings of your observations of the planet Mars is not as difficult as it seems. An observer typically produces a 42 to 50 millimeter blank disc in order to render and observation of the planet Mars.

The first and vital step in producing an accurate drawing of the planet Mars is a carefully rendered outline of the main albedo features (dark and light features)  visible over the disc of the planet.

An observer may decide to divide the visible disc into quarters (or eights) and place the albedo features visible within each quarter/eights. After careful review of the outline produced the observer begins the process of shading the darkest albedo features visible then gradually adding the more subtle shadings visible and finally rendering the lightest (brightest) regions of the planet (be careful to leave the brightest features of the planet (e.g. North or South Polar Caps and limb clouds) free of pencil marks.

The object of the drawing is not to produce a work of art but to render what is visible over the surface of the planet at a specific period of time as well as train the observer to detect fainter detail over time as they study the delicate surface and atmosphere of the red planet. The important thing is for the observer to make the process a fun one. 

• Mars
• Oppositions of Mars
• Mars in Retrograde
• Weather on Mars
• Observing Mars With a Telescope
• Learning to Draw Mars Albedo Features 

Ambassador Carlos Hernandez has contributed his planetary observations to worldwide organizations including The Association of Lunar and Planetary Observers (ALPO, United States), British Astronomical Association (BAA, Great Britain), Oriental Astronomical Association (OAA, Japan), and many other excellent planetary amateur astronomer groups over many decades.