A solar eclipse on Earth lasts only around 7 minutes and some of them can be total, annular, partial, or hybrid. The moon's shadow on earth is small compared to the Earth's shadow on the moon. Usually, the sky looks like twilight, and we only see the brightest planets in the sky during a total eclipse. We also would be able to see the sun's corona during a solar eclipse and would be quite dangerous to look directly.

Total Solar Eclipse Annular Solar Eclipse

However, a lunar eclipse viewing from Earth lasts around 2 hours and some are total, partial, or penumbral. The Earth casts a shadow on the moon, but because of the atmosphere, the moon turns red instead of total darkness.

Total Lunar Eclipse

What will be the difference of solar and lunar eclipses viewed from Earth if the moon was replaced by every planet in our solar eclipse assuming that the orbits are similar to the current Earth-Moon system? How big would the planet's shadow appear on Earth? How dark will daylight get during these events and what would the transition of an eclipse starting / ending look like? Would it still be dangerous to look at a solar eclipse when the planet blocks out the sun?

  • $\begingroup$ They'd be a lot less interesting. A solar eclipse makes the corona visible because the moon is about the same size, anything a lot bigger would just make it dark. Also a partial lunar eclipse still has the bright moon drowning out the red color, it's not until totality that you get to see the faint red. So anything larger would just have a black dot moving across. $\endgroup$ Commented Nov 7, 2022 at 21:50
  • $\begingroup$ The orbits couldn't be similar to the current Earth-Moon system. The Earth-Moon barycentre is inside the Earth, but an Earth-planet barycentre would be between the two bodies for the terrestrial planets, or inside the larger body in the case of the giant planets. See en.wikipedia.org/wiki/Barycenter#Gallery But I guess that doesn't really matter, and you just want the Earth-planet system to orbit the Sun in 1 year, and the Earth & planet to orbit their barycentre in 29.53 or so days, in a plane that's slightly inclined to their solar orbital plane. $\endgroup$
    – PM 2Ring
    Commented Nov 13, 2022 at 8:20
  • $\begingroup$ BTW, it is quite safe to view a solar eclipse during the time of totality, although it is certainly dangerous to look at it before or after totality, as I mentioned in astronomy.stackexchange.com/a/36630/16685 From mreclipse.com/Totality2/TotalityCh11.html "Once the Sun is entirely eclipsed, however, its bright surface is hidden from view and it is completely safe to look directly at the totally eclipsed Sun without any filters. In fact, it is one of the greatest sights in nature". $\endgroup$
    – PM 2Ring
    Commented Nov 13, 2022 at 12:44

2 Answers 2


There are a few ways to interpret "the orbits are similar to the current Earth-Moon system". We can keep the Sun-Earth and Earth-Moon orbit distances the same, or we can keep the orbit periods the same.

The orbital period equation for a two body system is

$$R^3 = \mu (T / 2\pi)^2$$

where $R$ is the semi-major axis, $T$ is the period, and $\mu$ is the sum of the standard gravitational parameters of the two bodies. That equation can be derived from the vis viva equation. (Also see Specific orbital energy).

For a small body orbiting a much larger body, we usually ignore the mass of the small body, but that's not very accurate when the bodies have similar masses. To keep the calculations simple, I'll assume that all the orbits are circular (and coplanar), so the orbit radius equals the semi-major axis.

I decided to keep the current Sun-Earth distance and to use the current sidereal period of the Moon for the period of the replacement moon. So we have the Earth and planet orbiting each other in a circular orbit, and the barycentre of that Earth-planet system orbiting the Sun in a circular orbit. Because the Sun is so massive, the year length of the modified system is fairly close to the current year. The biggest difference is (of course) with Jupiter as the moon, which gives a sidereal year of ~365.082 days.

However, the Earth-planet orbit radius changes significantly, especially with the giant planets.

Here is a table of the resulting Earth-moon orbit radii and angular diameters, as seen from the Earth's equator. The radii are given in units where the current Earth-Moon radius is 1. The angular diameters are in arc-minutes. I used the planets' polar radii for the angular diameter calculations.

Body Distance Diameter
Mercury 1.013955 43.72
Venus 1.214849 90.25
Earth 1.254797 91.74
Moon 1.000000 31.57
Mars 1.030397 59.51
Jupiter 6.803764 176.04
Saturn 4.562784 213.73
Uranus 2.485083 180.81
Neptune 2.617206 167.28

FWIW, the barycentres of these orbits are between the Earth and the other body, except for the Moon, Jupiter, and Saturn.

Here is the Python script which I used for those calculations. The gravitational parameters and polar radii came from Horizons body data. The script prints a few more values, including the year and synodic month lengths, and the distance of each body from the barycentre.

We can use those angular diameters to estimate the eclipse duration relative to eclipses in the current system. But we can't estimate the duration of totality without knowing the new Earth's day length. With the Earth orbiting a giant planet, it's likely that it would be tidally locked, so the day length would equal the month length.

The solar eclipses caused by Mercury and Mars would be similar to current solar eclipses, although some sunlight would be refracted by the thin atmosphere of Mars. The other planets have a lot of atmosphere, so they would refract a lot of light, which would create a similar effect on Earth to what we see on the Moon during a lunar eclipse in the current system.

The radius values for the giant planets are based on where the atmospheric pressure is equal to 1 bar, i.e., Earth's surface air pressure. There's still quite a lot of atmosphere at higher altitudes. And of course if the giant planets were only 1 AU from the Sun they would be significantly warmer, which would affect their atmospheres.

  • $\begingroup$ They's at least one more way to interpret the question. The way I interpret it is all of the kelperian elements stay the same, just the diameter of the object changes. $\endgroup$ Commented Nov 17, 2022 at 18:32
  • $\begingroup$ @GregMiller True, but I don't know how you'd get a planet the size of Jupiter with the mass of the Moon. I was hoping the OP would clarify the question in response to my comments... $\endgroup$
    – PM 2Ring
    Commented Nov 17, 2022 at 19:21

If the moon was as big as Jupiter then it would be comparable to holding a dustbin lid at arm's length, about 50cm at 1 meter. A current moon total eclipse would take 42 Times longer... 294 minutes, 5 hours complete darkness at totality. https://www.wolframalpha.com/input?i=jupiter+diameter+%2F+moon+diameter

Surface area would be 1900 times bigger, so total eclipses would happen every day at time of new moon for all the planet, 3-5 days a months.

Moon orbit is avg 5.1 degrees from sun ecliptic.

The change in the weight would make the orbit speed change considerably, our planet would have to orbit every hour otherwise it would crash into Jupiter. Tidal waves 150m high and very dark skies from constant volcanism.

For reference, Io is as big as the moon, has the same orbital distance, and has an orbit time of one day. https://en.wikipedia.org/wiki/Moons_of_Jupiter


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