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Right, you are standing on Earth 100 million years in the future and observing the sky. What would you see during night and how would a day look like when you look up. Weather is not relevant (no clouds), we can say the Earth's atmosphere is very similar to today's. And there is no light pollution.

I assume the Sun would be brighter... How much? And would our known constellation change? If so, how much of a change would you predict? And what about the Moon?

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  • $\begingroup$ I think this has been discussed before, with suggestions for software that might show the future star positions. $\endgroup$ – JDługosz Mar 19 '17 at 20:44
  • $\begingroup$ @JDługosz Well, I had some trouble finding an answered question similar to this one so I figured I'll just ask. Also, I am not interested solely in star positions, but in eeeverything above our heads :) $\endgroup$ – Raven Mar 19 '17 at 22:21
  • $\begingroup$ See this question. $\endgroup$ – JDługosz Mar 21 '17 at 9:05
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Sun

The sun is expected to increase its brightness by a factor of about 2.1 over the first 9 billion years of its life. Using extremely rough measurements, that works out to about 1% brightness every 100 million years. The Earth might be a bit hotter, in complex ways that probably depend a lot more on what we did with all that carbon and whether we humans are still around, but visually, the sun will be about the same.

Moon

Over the last hundreds of millions of years, the moon has receded from the Earth at 22 mm per year. Over 100 million years, that works out to 2200 kilometers. Since the Moon's current orbit is 385,000 km, that is a bit less that 1% farther away. If the Earth still has oceans, tides won't be affected much, and the moon will be slightly smaller in the sky. The moon is also causing the day to lengthen by about 12 microseconds per year, which works out to days that are around 20 min longer in 100 million years.

Stars

I haven't found any software that will let you look that far into the future at the constellations. However, the sky would be mostly unrecognizable even a few thousand years into the future, so on the timescale of millions of years we can expect the sky to be much different. For example, many of the brightest stars will no longer exist (Betelgeuse, for example, should supernova within the next few hundred thousand years and Rigel within the next few million years).

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    $\begingroup$ The Moon being 1% further away will have one noticeable effect, although not immediately noticeable. There will no longer be any total eclipses of the sun. $\endgroup$ – Mike Scott Mar 19 '17 at 21:32
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    $\begingroup$ CyberSky's list of features states: "View colorful, detailed maps of the sky as seen from any location on the Earth at any moment from 15,000 BC to 15,000 AD", so yeah... Also, I am not interested in detailed description of constellations 100 million years from now, I am more interested if somebody with basic knowledge of current constellations would be able to recognize something then, or would it all be in disarray. $\endgroup$ – Raven Mar 19 '17 at 22:00
  • $\begingroup$ @Raven Whoops, let me revise. $\endgroup$ – kingledion Mar 19 '17 at 22:09
  • $\begingroup$ Complete disarray. $\endgroup$ – JDługosz Mar 20 '17 at 0:54
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    $\begingroup$ @MichaelKjörling Here are the calculations showing that it's about 563 million years until we stop having total eclipses: spacemath.gsfc.nasa.gov/weekly/4Page28.pdf $\endgroup$ – Mike Scott Mar 21 '17 at 10:26
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Basically identical to this answer using an even larger number.

One lap around the galaxy is 250 million years. So 100 million is plenty of time for them to shift relative positions in a substantial way.

I think most of the visible stars are close, so travelling (mostly) as a group. So you’ll see a lot of the same stars, just moved in the sky and changed in brightness. Some stars will no longer be visible, and different ones will appear.

See proper motion in wikipedia, which includes links to software:

There are a number of software products that allow a person to view the proper motion of stars over differing time scales. Free ones include: …

Off hand, I think the appearance will be utterly and totally different.

See also this video by ESA’s Gaia and DPAC mission teams:

…the stars themselves will move. Combining positional data of unprecedented accuracy for two-million stars taken over years by ESA's Earth-orbiting Hipparcos (now defunct) and Gaia satellites, a future extrapolation of star movements was made over millions years. As shown in the featured video, many stars make only small angular adjustments, but some stars -- typically those nearby -- will zip across the sky. Once familiar constellations and asterisms will become unrecognizable as the bright stars that formed them move around. Not shown are many local nebulas that will surely dissipate while new ones will likely form in different places.

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  • $\begingroup$ So, if we, for instance, throw you in the year 100 000 000 AD, and you lie down in a future grass on a future hill and gaze into the future sky, would you understand that you are still on Earth? Would you be able to recognize anything above you? $\endgroup$ – Raven Mar 19 '17 at 22:14
  • $\begingroup$ @Raven not at all. $\endgroup$ – Joe Kissling Mar 19 '17 at 23:41
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    $\begingroup$ Propably the Sun, the Moon and the solar planets will still be there. $\endgroup$ – Julian Egner Mar 21 '17 at 11:31

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