# Cretaceous astronomy **URL:** \ **Category:** Factual Questions\ **Created:** [March 31, 2013, 2:31am UTC](https://boards.straightdope.com/t/cretaceous-astronomy/654399 "2013-03-31T02:31:42Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![Lumpy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lumpy/32/446_2.png) [@Lumpy](https://boards.straightdope.com/u/Lumpy)\ **Post date:** [March 31, 2013, 2:31am UTC](https://boards.straightdope.com/t/cretaceous-astronomy/654399/1 "2013-03-31T02:31:42Z")
Back in the late Cretaceous, about 65 M years ago: 1. How much closer to the Earth was the Moon? Would it look appreciably bigger to the naked eye? 2. How long was the day? How many solar days in a solar year? Ditto, how many days in a lunar month? 3. Was the Earth’s axial tilt different from the current 23.5 degrees? Was the Earth’s orbit more elliptical than today or less? 4. Do we have any idea what stars and constellations we’d have seen, or is it too long ago to track star movements? ---
**Author:** ![Chimera](https://avatars.discourse-cdn.com/v4/letter/c/8e8cbc/32.png) [@Chimera](https://boards.straightdope.com/u/Chimera)\ **Post date:** [March 31, 2013, 2:46am UTC](https://boards.straightdope.com/t/cretaceous-astronomy/654399/2 "2013-03-31T02:46:21Z")
On the star movement thing, we revolve around the Milky Way every 200 million years, so that was 1/3 of a revolution ago. If you look [here](http://en.wikipedia.org/wiki/List_of_nearest_stars) at the list of nearest stars, there is a [chart](http://en.wikipedia.org/wiki/File:Near-stars-past-future-en.svg) which gives you some idea of their relative movement over even a relatively short time compared to the 65 million year time frame you ask about. Thus I would venture to say it would be nearly impossible to determine what the constellations were like that far back. ---
**Author:** ![engineer\_comp\_geek](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/engineer_comp_geek/32/504_2.png) [@engineer\_comp\_geek](https://boards.straightdope.com/u/engineer_comp_geek)\ **Post date:** [March 31, 2013, 3:15am UTC](https://boards.straightdope.com/t/cretaceous-astronomy/654399/3 "2013-03-31T03:15:05Z")
The moon is moving away at something like 4 cm per year, which over 65 million years would be 260 million cm, or 2600 km. The diameter of the Moon is 3,474 km (according to some random site I googled) so it has moved outward less than its own diameter. It’s not going to look noticeably different. ---
**Author:** ![cmyk](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cmyk/32/3353_2.png) [@cmyk](https://boards.straightdope.com/u/cmyk)\ **Post date:** [March 31, 2013, 4:26am UTC](https://boards.straightdope.com/t/cretaceous-astronomy/654399/4 "2013-03-31T04:26:45Z")
> [@Chimera](#): > > On the star movement thing, we revolve around the Milky Way every 200 million years, so that was 1/3 of a revolution ago. If you look [here](http://en.wikipedia.org/wiki/List_of_nearest_stars) at the list of nearest stars, there is a [chart](http://en.wikipedia.org/wiki/File:Near-stars-past-future-en.svg) which gives you some idea of their relative movement over even a relatively short time compared to the 65 million year time frame you ask about. Thus I would venture to say it would be nearly impossible to determine what the constellations were like that far back. Looks like, according to that chart, Proxima/Alpha Centauri will only be about 3 LYs away instead of over 4 in about 25,000 years. If we leave now, we’ll get there sooner rather than later! Wait… uh… lemme think about that one… ---
**Author:** ![Spectre\_of\_Pithecanthropus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/spectre_of_pithecanthropus/32/12343_2.png) [@Spectre\_of\_Pithecanthropus](https://boards.straightdope.com/u/Spectre_of_Pithecanthropus)\ **Post date:** [March 31, 2013, 6:18am UTC](https://boards.straightdope.com/t/cretaceous-astronomy/654399/5 "2013-03-31T06:18:25Z")
> [@Chimera](#): > > Thus I would venture to say it would be nearly impossible to determine what the constellations were like that far back. I’m not so sure about this. Most stars in the galaxy are much smaller and much fainter in absolute terms than the sun; in this [list of the nearest stars](http://en.wikipedia.org/wiki/List_of_nearest_stars#List), the overwhelming majority are much to faint to be seen by the naked eye. The few of those stars that are bright in our sky, like Sirius, Alpha Centauri, and Procyon are very much the exception; they are bright to us not only because considerably brighter than the average dwarf star, but also because the vagaries of galactic formation placed them extremely close to us. OTOH look at thislist of the brightest stars as seen from Earth. Many of these stars are hundreds of light years away. I don’t think their apparent change in position will be that much, but I’m only making an educated guess. I don’t know but what I might install Celestia again and see if it will let me go back to the Cretaceous and confirm it for myself. ---
**Author:** ![snailboy](https://avatars.discourse-cdn.com/v4/letter/s/97f17d/32.png) [@snailboy](https://boards.straightdope.com/u/snailboy)\ **Post date:** [March 31, 2013, 8:05am UTC](https://boards.straightdope.com/t/cretaceous-astronomy/654399/6 "2013-03-31T08:05:57Z")
> [@Spectre\_of\_Pithecanthropus](#): > > I don’t know but what I might install Celestia again and see if it will let me go back to the Cretaceous and confirm it for myself. Celestia is pretty cool but I’d be surprised if it accurately shows the movement of stars over millions of years. ---
**Author:** ![eburacum45](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eburacum45/32/8690_2.png) [@eburacum45](https://boards.straightdope.com/u/eburacum45)\ **Post date:** [April 2, 2013, 1:53am UTC](https://boards.straightdope.com/t/cretaceous-astronomy/654399/7 "2013-04-02T01:53:07Z")
Celestia doesn’t show the proper motion of stars at all (although it does show the orbits of some closer binary stars around each other). To see proper motion you want a different program. Stellarium for example. But no program can accurately predict where the stars were 65 million years ago.