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?
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.
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.
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).
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.
