Other total eclipses in the Solar System?

2024 December 9

Stephen J. Edberg

An extensive survey of the 290 known satellites of the planets and (134340) Pluto, current to 2023, was made to identify host planets potentially treated to annular and total solar eclipses matching the magnitude ranges of these events observed on Earth. Earth’s eclipses are, indeed, unique in the Solar System. Among those six outer bodies, astronauts on the surfaces or in aero-vehicles above their cloud tops would have opportunities to see over-occultations and/or transits of the Sun due to their satellites. In addition, astronauts on the surfaces of some satellites orbiting Jupiter, Saturn, and Uranus would have the opportunity to observe annular and total solar eclipses. Appendices name and define alignments in the Solar System and describe how the survey was carried out, including its limitations.

Introduction

It has been said, for decades or longer, that Earth is the only planet in the Solar System that has total solar eclipses. But is that true?

Yes, with qualifiers. Earth’s total eclipses (henceforth ‘total eclipses’) are unique in the Solar System, among the planets having satellites. It is easy to think they are unique because:

(1)          They have a uniquely good ‘fit’ over the Sun by the Moon.

(2)          The eclipses appear relatively large in the sky (because Earth is relatively close to the Sun).

(3)          They last a relatively long time, usually counted in minutes. (Although few eclipse chasers would say a total eclipse lasts a long time.)

In fact, only item (2) is unique. This study demonstrates that even by limiting the definition of annular and total to the magnitude range of these events on Earth, there are events that match item (1) and some that can match item (3). This survey also identified satellite transits that range from ‘near annular’ to matching the lowest magnitude of Mercury’s transits observed from Earth.

Testing planetary satellite systems for total eclipses, and related annular (ring) eclipses, can be accomplished at the same time. The number of potential syzygies, though, is sobering. In 1965, 31 natural satellites were known in the Solar System. Many more satellites have been discovered since Earth’s three crossings of Saturn’s ring plane in 1966, because of improvements in detectors, increased telescope time allocations for satellite and deep Solar-System studies, wide-area sky surveys that have been ‘mined’ for moving foreground objects in the vicinity of outer planets, and spacecraft exploring the planets. By mid-2023, the count of satellites among the planets and (134340) Pluto numbered 290 objects, some still awaiting formal designation by the International Astronomical Union. Not included in this study are 494 asteroids and trans-Neptunian objects (including Pluto), with 514 satellites.¹

One can use a shorthand approach to quickly evaluate all the satellites, by determining their angular size and that of the Sun as seen from their host planet. Variations in the distances of the planets from the Sun and the satellites from their hosts occur because each planet’s orbit and each satellite’s orbit have different degrees of ellipticity. In the case of Earth and the Moon, we see more annular than total eclipses because of their elliptical orbits.

On Earth, a total eclipse is defined to have a magnitude: the ratio of Moon’s angular diameter (or radius) to the Sun’s angular diameter (or radius), greater than or equal to 1.00. With the present orbits of Earth and the Moon around the Sun, the maximum value of eclipse magnitude can reach about 1.087 (the Moon’s apparent diameter being about 9 per cent oversized compared to the Sun). On the other side, an annular eclipse has a magnitude less than 1.00, down to about 0.939 (the Moon’s silhouette diameter being about 6 per cent smaller than the solar disc).

In more general terms, an occultation occurs when an observer sees a body with a larger angular size covering a body with a smaller angular size (as happens during a total solar eclipse or an occultation of a star by the Moon). A transit occurs when a smaller-appearing object crosses in front of the visible hemisphere of a more distant, larger-appearing object (as happens with an annular eclipse or a transit of Mercury, Venus or the International Space Station across the Sun). Jupiter’s large Galilean satellites and their shadows make frequent transits of Jupiter.

For observers on a planet’s surface or above its cloud tops, its satellites can offer transits, potentially annular and total eclipses, and/or over-occultations, with the satellite’s apparent disc much larger than that of the Sun. Which of these is visible depends on:

–  How far the satellite is from its host planet. This determines the apparent angular size of the satellite.

–  What projection of the satellite’s ellipsoidal volume (ranging from potato-shape for small satellites to spherical for larger satellites) is presented to the observer on a planet’s surface or cloud tops. In other words, the shape of the silhouette seen crossing the Sun’s face.

–  Where the planet is in its elliptical solar orbit, which determines the angular size of the Sun that is seen behind the transiting or occulting satellite. The Sun’s angular diameter changes with a host planet’s orbital distance.

A planet’s inner satellites and their circular or near-circular orbits are found generally close to their host. It is common for the Sun to be over-occulted by these ‘regular’ satellites of the large outer planets and Pluto. Small inner satellites may be close enough for total or annular eclipses but may produce events unseen on Earth if the satellite is elongated.

Transits & solar eclipses viewed from the planets

Throughout the Solar System, transits by planetary satellites are sunward syzygy events (Sun-satellite-planet alignments). Some potential events seen from the surfaces or cloud tops of the planets come close to central solar eclipses that we see on Earth’s surface (and up to the stratosphere in aircraft). With a few exceptions mentioned below, these events are based solely on the mean radius of the satellite (see Appendix 2). Using mean radius to determine the magnitude of a transit immediately indicates the likelihood of possible annular or total eclipses visible on the host planet.)

Because the eclipse magnitude range spanning 0.939 to 1.087 encompasses all total and annular eclipses on Earth, we can consider magnitudes in this range to be necessary, but not sufficient, to identifying similar possible eclipses on other planets that match the views of Earth’s annular and total eclipses (though not the angular sizes or durations of Earth’s events). The initial survey of all the satellites, using their mean radii, immediately showed that, with the exception of II Nereid orbiting Neptune, none of the irregular satellites (having elliptical orbits, in or out of the plane of the host planet’s equator) ever get close enough to its host to do more than transit the Sun. Most of those transits have magnitudes less than the minimum magnitude of transits of Mercury seen from Earth, 0.005.

For the regular satellites, the next step is to look at the actual long, intermediate, and short axes, as opposed to the mean radius, of the bodies and determine their eclipse magnitudes.

A spherical satellite having eclipse magnitudes greater than 0.939 and less than 1.087 will offer views like those seen on Earth. For any ellipsoidal satellites whose intermediate and short radii both offer eclipse magnitudes within that range, the satellite’s orbit’s orientation and the satellite’s rotation must be known to determine what the eclipse will look like and where on the planet (or its cloud tops) the view would be similar to Earth’s.

Except for the Moon, none of the known regular satellites provide total or annular solar eclipses on their host planets, though transits and over-occultations abound. Still, the cases described here show that in the Solar System, nature provides a far more imaginative tapestry than the simple thread of only Earth having total solar eclipses.

Mercury and Venus, lacking natural satellites, will not observe solar occultations. However, observers above the clouds of Venus can observe transits of Mercury at a slightly higher rate than we see them from Earth (Figures 1 & 2).

On Earth, the Moon provides total eclipses (Figure 3), annular eclipses (Figure 4), and hybrids, due to Earth’s surface curvature.

Treating Earth as a satellite of the Moon (this is not so far-fetched, since the Moon actually orbits the Sun, not Earth; see Appendix 1), the Earth over-occults the Sun by a ratio of about 3.7 (Figure 5). Two views of Earth during a total lunar eclipse (as seen from Earth) were taken by Surveyor 5 on the lunar surface. The two images downlinked showed changes in the atmospheric halo surrounding Earth and refracting sunlight around Earth’s terminator to the lunar surface. Most of the change was due to the shift in the Sun’s apparent position behind Earth over the course of time. Colourised versions of these images can be found.

Mars’ satellites Phobos and Deimos are only seen to transit the Sun (Figure 6) from a limited band on Mars’ surface. The band shifts with Mars’ obliquity-seasons. (‘Obliquity’ refers to the tilt of a planet’s geographic, rotation poles with respect to its orbital plane.) Phobos and Deimos are so close to Mars that they are always below the horizon outside of the band, which is centred on the Martian equator during Mars’ equinoxes. The magnitude range of Phobos’ transits is 0.55–0.68. Deimos’ transit magnitudes range from 0.09–0.11.

Of Jupiter’s large Galilean satellites, only distant Callisto comes close to providing total-like eclipses that could be seen from Jupiter’s cloud tops. But its angular radius is oversized by 40–57 per cent. Those total occultations of the Sun are only visible around Jupiter’s equinoxes. Even with Jupiter’s low obliquity, Callisto mostly passes above or below the Sun as seen from Jupiter in Callisto’s ‘new moon’ positions.

Among Jupiter’s smaller companions, when non-spherical Amalthea (Figure 7) passes in front of the Sun it offers the possibility of a thick-ring transit of an irregular shape when viewed along its long axis. Potentially, a ‘sliding bar’ that extends beyond the Sun as it goes by might be seen above the clouds at Jupiter’s higher latitudes or its sunrise and sunset terminators. Amalthea’s axes are 250, 146, and 128 km in length. These correspond to eclipse magnitudes of 1.19–1.32, 0.70–0.77, and 0.61–0.68, respectively.

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