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Comment by divbzero | Hacker News Reader

A curious effect of this is that total solar eclipses past and future look different from total solar eclipses today. We live in a time when both the Sun and the Moon are roughly the same size (~32 arcminutes) when viewed from Earth. This coincidence results in the image we associate with total solar eclipses where the Sun’s corona is visible as a ring around the Moon.

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A central plot point in Iain M. Banks' novel 'Transition'

How many years would this be the case? I mean how far back would we have to go to NOT see them be the same size

Roughly the same size is understating it severely. They are uncannily similar in apparent size.

The surface of the Earth is the only place we know of where a natural satellite appears exactly the same size as the primary / only star.

This can't be true: you can choose a point behind any sufficiently round satellite where it appears the same size as the star.

Perhaps it's the only place we know of on a planet's rocky surface. But most of our solar system's planets with moons don't have rocky surfaces, and we can't detect moons in other systems, so you really mean it's not true from the surface of Mars, Pluto or Charon.

Additionally, the moon varies in apparent diameter by about 13% from apogee to perigee, and the sun by about 3%. So while it might appear "exactly" the same size at some point in its orbit, mostly it's just within 5%.

Yes, sorry, I should have also wrote "where one can stand".

The Moon Kerberos of Pluto is actually very very close to the size of the sun in the sky (of by being 0.0099° compared to the suns 0.0089°, around 11%), so the earth isn't the only planet (if you count Pluto as planet, otherwise, but it's still a solar body you can stand on where a moon is roughly the size of the sun).

Doesn't Pluto have some weird elliptical orbit? Does it not matter enough to change the apparent size of the sun as seen from Pluto's surface?

I used the current values, they should be valid for a reasonable timeframe.

Edit: only by a few percents; the distances are too large to appreciably change the visible size of the sun in the sky.

We are actually pretty close to having found "exomoons"(natural satellite orbiting an exoplanet), with several promising candidates having been identified already.

David Kipping's Cool Worlds lab at Columbia has done some promising research on this, some of which is detailed in excellent videos on their youtube channel.

In even 1000 years that would only be an offset 38 meters. That wouldn't meaningfully change the ratio.

While not overwhelming an orbit change of 38000km per 1b years sounds more than enough to change that image.

38000km is just 10% of the current Moon-Earth distance and on the time scale of billions years, there are other things to worry: "the Sun will likely engulf Earth in about 7.59 billion years." https://en.wikipedia.org/wiki/Future_of_Earth#:~:text=Earth%....

And another factor will be the Sun growing in size due to hydrogen exhaustion: https://en.wikipedia.org/wiki/Sun#After_core_hydrogen_exhaus...

But at the time also the Earth's average distance from the Sun will grow, wouldn't it?

No, it won't. Earth will either be a scorched planet or will be engulfed by the Sun. And it won't take those 5 billion years either for life to vanish from Earth. We're currently near the inner edge of Goldilocks zone (Mars is just a bit outside of the outer edge) and we have only like 500 million years. Either start colonizing other planets or we die, there nothing in between (assuming we don't destroy ourselves meanwhile).

Third option, we can take advantage of repeated gravitational slingshots to transfer orbital kinetic energy from Jupiter to Earth via an asteroid going repeatedly between both planets, gradually increasing Earth's orbit.

(I'm not an astrophysicist, I just read about this idea a few years back and it stuck in my mind).

I guess Jupiter would be more efficient but it would be really neat to do this to mars bringing it nearer to the habitable zone/earth

The limit on earth's habitability is determined by permanent sequestration of atmospheric CO₂ through the carbonate-silicate cycle, not insolation. Once CO₂ drops below a certain level, photosynthesis will no longer be possible and all remaining ecosystems will collapse.

(Don't mistake that as an endorsement of burning fossil fuels — climate change is operating at a rate measured in decades, CO₂ drawdown via the carbonate-silicate cycle operates at a rate measured in hundreds of millions of years)

True, I had forgotten that. But I assume we can fix that over this kind of timescale.

While I am not generally a proponent of waiting until the last minute to worry about a problem, I think with a time frame of hundreds of millions of years, we can afford to procrastinate a bit.

Given how far we've come in the past 150 years, no sense in waiting for an existential threat. Let's get crackin'!

While sun would grow in size, it would also be losing mass. This reduces the centripetal force of revolution. The orbit would indeed expand.

The loss of mass is insignificant and we are talking 0,5B years.

Sun will be loosing a lot of mass later, but that will be way after Earth has been baked to a crisp.

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