Alachua Astronomy Club: Apparent Size of Mars

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	Disk of Mars	Member Astronomical League	Member International Dark-Sky Association
	29°39' N, 82°21' W Altitude: 50 Meters (more or less)		Updated July 29, 2005
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Why the Martian Disk Changes its Apparent Diameter

THE DISK OF MARS changes its apparent diameter (see Fig. 1) depending on its distance from Earth.

Mars appears smallest (Fig. 1a) when on the other side of the Sun from Earth (see Fig. 2). This position is called conjunction.

Mars appears largest (Fig. 1b-d) when "opposite" Sun's position in sky (also see Fig. 2). This position is called opposition.

The Martian orbit is about 1.5 times larger than Earth's orbit (see Fig. 2).

When Mars is farthest from Earth (conjunction) it can be over five times farther from us than when closest (opposition)! This also makes the disk of Mars seem to change by over five times! (See Fig. 2.

Can you see why the distance between Earth and Mars can change by over five times from Fig. 2? (Click on Fig. 2 to find answer.)

You can see the remarkable difference by comparing the image sizes in Fig. 1a when Mars is closest to Earth and Fig. 1c when Mars is at an "average" opposition distance.)

Nevertheless, because the Martian orbit is non-circular, some oppositions place Mars farther away than at other oppositions.

If Mars is at opposition and also near its farthest point from the Sun (called aphelion), we have an "unfavorable opposition" (see Fig. 3). Then the disk of Mars appears smaller (see Fig. 1b) than at an "average opposition" (Fig. 1b).

But if Mars is at opposition and also near its closest point to the Sun (called perihelion), we have a "favorable opposition" (see Fig. 4). Then the disk of Mars appears at it largest possible value (see Fig. 1d).

The Earth's orbit is also slightly non-circular but only about one-sixth as much as the Martian orbit. So, this makes some favorable oppositions even "more favorable" than others.

This happened in 2003 since the Earth reached its 2003 aphelion (farthest distance from Sun) on July 4, 2003 with the Martian perihelion date only eight weeks later (August 29).

Thus, the year 2003 produced the closest approach of Mars in nearly 60,000 years (Fig. 1d)!

The next opposition (2005 November) produces a maximum apparent size of about 20.2 arc sec, or slightly greater than the disk shown in (Fig. 1c).

Finally, a 20 arc sec disk is about 1/90 the apparent size of a Full Moon, which has an apparent size of about 1800 arc sec (or 1/2 degree).

Fig. 1. The Changing Apparent Diameter of Mars. Numbers in parentheses give the angular diameter of the plant in arc seconds (one arc second = 1/3600 of a degree)

[Click above image to see a Mars animation]

INTERNET MYTH

A RUMOR spreading through the Internet states the disk of Mars at the 2005 opposition will be as large as the Full Moon!

FALSE!

As noted previously the Martian disk will appear only about 1/90 the apparent size of the Full Moon when near the 2005 November opposition. (Mars appeared about 1/75 as large as the lunar disk at the 2003 August opposition.)

So, if one used a telescope with a magnification of about 90x, the image of Mars in the telescope would appear about the same size as the Full Moon appears without a telescope.

For more information about this myth, see "Mars Hoax"

Note: The Mars hoax message also contains other misleading information, especially material on the 2003 opposition but published as if applying to the 2005 opposition.

For an excellent animation showing the orbits of Earth and Mars including the changing diameter of the Martian disk as seen from Earth,
see The Orbit of Mars from Windows to the Universe

Figures 2 – 4
[Open in a Separate Window]

Distances shown in Fig. 2–4 are in AUs. Astronomers measure distances in the Solar System in Astronomical Units (AU)
where one AU is the average distance of the Earth from the Sun (about 93,000,000 mi or 150,000,000 km).

Fig 2. Orbits of Earth and Mars
(Click to enlarge in separate window)

Mars is farthest from Earth at conjunction and about five times closer to Earth at opposition

Fig. 3. Unfavorable (Aphelic) Opposition
(Click to enlarge in separate window)

An opposition when Mars is farthest from the Sun produces an unfavorable opposition

Fig. 4. Favorable (Perihelic) Opposition
(Click to enlarge in separate window)

At favorable oppositions Mars is closest to Sun & appears 2x larger than at unfavorable oppositions

Go to AAC Mars 2005 Special Public Event Page

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