Martian Transit Phenomena: Why Phobos and Deimos Fail to Produce Total Solar Eclipses for Surface Observers

Phobos and Deimos are too small for total eclipses on Mars. Learn how these solar transits help NASA rovers refine orbits and navigate the Martian surface.

By: AXL Media

Published: Feb 28, 2026, 5:06 AM EST

Source: The information in this article was sourced from Astronomy

Geometric Constraints of the Martian Lunar System

The physical dimensions of the Martian moons fundamentally prevent the occurrence of total solar eclipses as experienced on Earth. According to Astronomy Staff, Phobos and Deimos possess diameters of only 12 miles and 6 miles respectively, which are too small to blot out the Sun from the perspective of a surface observer. While the Sun spans approximately 21 arcminutes in the Martian sky, Phobos covers only 11 arcminutes and Deimos a mere 2 arcminutes. Consequently, these events are classified as transits rather than true eclipses, as a significant portion of the solar disk remains visible even during peak alignment.

Atmospheric Impact and Visual Characteristics of Transits

Although they lack the totality of Earth based eclipses, transits of the Martian moons significantly alter the environment on the planet's surface. According to the report, a transit of Phobos can cause solar intensity to drop by as much as twenty-five percent, creating a visual effect similar to an annular or partial eclipse. In contrast, Deimos transits are much more subtle, appearing as small dots crossing the Sun in a manner reminiscent of the rare transits of Venus seen from Earth. These events occur with high frequency depending on the latitude and season, typically appearing several times within a two week window.

Navigational Utility of Shadow Scanning

The predictable movement of lunar shadows across the Martian disk serves as a vital tool for planetary navigation and site location. According to the analysis, the shadow of Phobos effectively scans the surface, moving progressively southward following the winter season in the northern hemisphere. This phenomenon was intended to assist the Beagle 2 lander in pinpointing its landing coordinates by observing the specific timing of Phobos transits. By measuring exactly when the shadow passed over the vehicle’s latitude, researchers could theoretically calculate the lander's position with high precision relative to the Martian equator.