The Science of Sustainability

Luna Nova: Moon of the Cretaceous Skies

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Moon today and 85 million years ago, during the Cretaceous

Moon today and during the Cretaceous

Although I am a lifelong fan of science, I've also been a lifelong fan of science fiction—so I sometimes experience conflict in the DMZ where the two meet.

Having been raised on Star Trek, where the science and technology routinely violate known scientific principles (faster than light warp drive, for example), I learned to have leniency on some of those violations—at least, the ones that exist in order to make the story work.

But the stories that get the science completely wrong, for no good reason, get my militia up in arms….

Such was my reaction when, a few weeks ago, I happened upon the last two minutes of the series premiere of a new television show—the one that involves time-traveling colonists going 85 million years into the past to live among the dinosaurs. (Don’t ask me any more about the plot; I’ve only ever caught the last two minutes of each show when I change the channel to wait for House. All I know is each episode seems to end with people creeping through a jungle at night carrying torches….)

So what irked me so badly? Scene: colonists in settlement in Cretaceous jungle, night time, looking up at the starry, Moon-adorned sky. A child muses, "Is that the Moon?" (never having seen it before). "It’s so big!" Indeed, the Moon aloft in these prehistoric skies was depicted as truly huge—I’d estimate ten or fifteen degrees across, about the width of your hand spread wide at arm’s length (20 to 30 times the size of the Moon we know).

Enter "brainy" teenage girl to explain: The Moon is moving away from the Earth a few centimeters each year, so here, 85 million years in the past, it’s much closer to Earth.

How much closer was the Moon to Earth 85 million years ago? Do the math, brain: The Moon is currently moving away from the Earth at about 3.8 centimeters per year, so 3.8 cm for 85 million years equals 323 million centimeters. Sounds like a lot, right? 323 million of just about anything seems like a lot. 323 million centimeters is 3,230,000 meters, or 3,230 kilometers. Or a little over 2,000 miles—which, coincidentally, is about the diameter of the Moon itself. Since the Moon is presently 240,000 miles from Earth, being 2000 miles closer to us in the past (about 0.8%) would not have made it perceptibly larger—let alone appearing as big as a cantaloupe!

The Moon has been moving away from the Earth since its formation, which took place about four and a half billion years ago. Through tidal interactions with the Earth, the Moon has "stolen" some of Earth’s rotational momentum (spin) to gradually boost itself farther and farther away, slowing the Earth’s spin as a result. Back in the day when the Earth and Moon were young and fresh—and much closer together—the Earth spun much faster: maybe once in 8 hours. (But that was WAY before life existed, so try not to imagine the dinosaurs experiencing much shorter days, please.)

Oh yeah, in that same two minutes of the show premiere, the "brainy" girl (it’s not her fault; it’s the show’s writers, of course) also had an answer for why all the stars in the Cretaceous sky bore no resemblance to the constellations we know today. The Universe is expanding, she said (correctly), and so in 85 million years that expansion has caused the stars to change position" (not so correctly). The Universe is expanding, yes, correct; the stars in Earth’s skies 85 million years ago would have looked completely different, yes. But the two have nothing to do with each other.

The Universe is expanding and carrying all of the galaxies and galaxy clusters farther and farther apart. But this has no effect on the stars gravitationally bound within each galaxy. At the scale of a single galaxy, like our own Milky Way, the gravity binding the stars together in that great spinning spiral overpowers the effect of space expanding.

The stars we see in our skies are all inside of our galaxy, to which they are gravitationally bound. It is merely the motion of those stars within the galaxy as they orbit the center that change their relative positions, and so the patterns of constellations that we perceive. Analogously, continental drift on Earth may move a pair of land masses away from each other, but that large-scale motion won’t cause the trees within either of those lands to move apart.

Nit picking? Yeah, maybe. But I even do it to Star Trek on occasion….

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Category: Astronomy

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Ben Burress

About the Author ()

Benjamin Burress has been a staff astronomer at Chabot Space & Science Center since July 1999. He graduated from Sonoma State University in 1985 with a bachelor’s degree in physics (and minor in astronomy), after which he signed on for a two-year stint in the Peace Corps, where he taught physics and mathematics in the African nation of Cameroon. From 1989-96 he served on the crew of NASA’s Kuiper Airborne Observatory at Ames Research Center in Mountain View, CA. From 1996-99, he was Head Observer at the Naval Prototype Optical Interferometer program at Lowell Observatory in Flagstaff, AZ. Read his previous contributions to QUEST, a project dedicated to exploring the Science of Sustainability.
  • Sfroofdog

    That must mean a total solar eclipse covered the Sun more 85 million years ago. In 85 million years, perhaps a total solar eclipse won't be possible because the Moon will be too far away?

  • http://www.facebook.com/stephen.yang Stephen Yang

    The problem with time travel is that you have to know WHERE something was X million years ago. The Earth not only rotates on axis, but orbits the Sun in a solar system that is also moving in the Milky Way galaxy. You can't just simply go backwards in the single dimension of Time. You still have 3 other dimensions to calculate for.

    If you're going to stay in one spot and go back in time for even just a year, odds are you're just going to appear in the void of space.

    • Mike

      In most shows, time travel happens via wormholes. If those can happen at all, physics tells us that they likely end in physical structures ("negative mass rings" or some such thing). Those physical structures would travel along through space with whatever planet they were on. The idea that time travel would leave you "where the earth was when you left" really makes very little sense.

  • http://www.facebook.com/ronald.stepp Ronald Stepp

    You know, nitpicking aside, how neat would it be if the moon really WERE that close back then and it turns out some event (aliens, cosmic event, etc) caused it to move much further out where it is today?

  • Fredtobik

    The only problem I have with it, is that someone is out there telling someone else false information. I assume people don't believe everything they see on T.V., but you know what happens when you assume? ass-u-me.

  • Mj221

    You can suspend your disbelief a little more. A reasonable interpretation of what happened is that the viewer just fell prey to the usual visual illusions making the moon appear much larger, and then came up with an incorrect explanation. Both of those happen all the time. Presumably, even in the future (or past), not everybody is perfect.

  • Sky Watcher

    Good article. Does this fiddling with the facts take away from the romance of the moon? Do the less intelligent speculators enjoy the moon more than highly trained astronomers? When do we reach the point of "that's just the way things are," like when a child asks a parent about sex?

  • http://profiles.google.com/sol.europa Bill C

    well the rate the moon is travelling away is increasing (it's not been a steady 3.8 since it was formed! it was around 2cm/y 135 – 65 m years ago)
    Also the moon looks 12% larger between apogee and perigee.

    This is not getting into the whole "alternate time line" aspect of the show.
    It was explained in the first episode that they are not in the same time line as the one they left. (aka "Many-Worlds" / "Parallel timeline" interpretation of time travel).

    Also theirs is the popular "Poetic licence" film makers use i.e telephoto lens to shoot shots of the moon to make it look larger.

  • Stephen Mann

    While the Moon does recede from Earth because of tidal friction between it and that, the Sun has almost half the Moon's tidal effect upon Earth, as well. I don't believe the Moon moved away from Earth at that time, however, even though tidal interactions existed then. At that time, the rotation of Earth's molten core was faster than now and it transferred its surplus to Earth's mantle, thus compensating for tidal friction. One tipoff to me that this is so is the ratio of Earth's circumference to the Moon's: 3.65 or its year divided by one hundred; the reverse of this is .27, or the month divided by one hundred. This indicates both have been locked into an orbital-rotational reciprocity since their "creation" 4.57 billion years ago. Of course, this contradicts current theory. I also think Earth recedes from the Sun now at the rate of twenty-three to forty-nine feet per century ("Astronomy", Jan. 2009), compared to the Moon's recession of eight to twelve feet per century. Now that Earth's core cools, it no longer compensates for tidal friction upon its crust. Also, Earth was closer to the Sun millions of years ago, which is why it was warmer. At its closest to the Sun, the year was only 360 days (fifty-five million years ago). But the Moon has been about 242,000 miles on average since "creation".

  • Stephenmann35

    Observations made of distant galaxies indicate they were denser when younger and have since expanded. Andromeda and the Milky Way now approach each other rather than recede. Since they were once united and are now apart, they must have at first receded from each other but then reversed. Therefore, the greater universal expansion must have reversed itself locally on the galactic and neighboring galaxy level. Why would Andromeda and the MW now approach each other rather than recede? I think their magnetisms must have opposed each other until the less massive of the two (the MW) inverted to Andromeda to create complimentarity and allow their mutual approachment.

    • Nate

      They are approaching each other now because of their gravitational effect on each other. When they were "united" as you mention (I assume you are saying this because of the big bang theory, everything was in one place so therefore they were together), they were not galaxies. They were a hot plasma soup of particles. As the universe expanded and cooled, and due to the initial quantum fluctuations of the plasma soup when it was much much much much much smaller, gravity took hold and started making these two galaxies clumpy. Eventually stars formed, etc etc etc. These two galaxies were never in the same place in their current state. That is, as spiral galaxies. They picked up their angular momentum from collisions with impact parameters of varying size (off axis mergers). Once they got bigger, they started to pull on each other more and more, and they are now on a collision course. Don't worry though, the largest effect this could have on the solar system level would be to get launched out of the (then composite) galaxy with a bunch of other stars. Mergers are messy. Here is a beautiful video:

  • http://twitter.com/GabrielRoybal Gabriel Roybal

    hard to spot the differences!

  • Bklynindy

    You had me until, "The stars we see in our skies are all inside of our galaxy…" Not true. Many of the "stars" we see in our skies are in fact other galaxies, so distant that they look like stars. Definitely not in our galaxy.

    • Ben Burress

      Actually, no. Every individual star you can see, with your unaided eye, is inside the Milky Way galaxy. We can see three or for extragalactic objects with our unaided eyes (such as the Andromeda Galaxy, and the two Magellanic Clouds), but we cannot perceive their individual stars, just the diffuse glow of their massed stars.

  • 6bird4

    At 2000 mil closer would not this effect the tides to much to have life on earth.

  • Brent

    I am still trying to get my brain around that something came from NOTHING. It was about the size of a pin head or fingernail, and 200 billion GALAXIES with 100 billion stars in each galaxy (+/-) blosomed from that pin head 'explosion'.
    Wow! Faith is alive.

    • Bruce Wayne

      It is not faith to acknowledge that our observable universe is expanding from a central point. That is something we can see and collect data on. From there we can extrapolate that everything was once very tightly packed together.

      Just because we don't have all the answers doesn't mean the ones we do have are invalid. If you go outside and you see a tree, and every day you record the height, and eventually you notice it is getting taller, you can make a pretty solid guess that it used to be much smaller. Maybe you even figure out that it came from a seed, based on how other trees behave. So this is your information, and you use it to figure out more information, and if some parts of your information are wrong, you go back and correct it.

      You know what requires faith? Deciding that some magical being just made everything on a whim. You know what doesn't make sense? Ignore the mechanics of your reality in favor of a children's story.

  • alex92262

    I just read an article that indicates the rate the moon is moving away from earth is slowing, and that eventually that will stop. So what if that is the case? Then the 2 cm rate you use is an incorrect assumption. And, based on calculations about the collision effects with a smaller planet ("Thea?), the formation of the moon could not have happened as it did with the fragments coming together if the distance was MORE than five times the size of the earth, or less than 3 times the earth…. so the speculation is that it was somewhere between 10 and 30,000 km – roughly 1/30 to 1/10 what the distance currently is. I think like anything else, calculations made in a vacuum are terribly misleading, and hence, your frustration seems unfounded.