365DaysDate: March 28, 2009


Title: What if the Moon did not exist?

Podcaster: Patrick McQuillan

Organization: Incorporated Research Institutions for Seismology (IRIS)

Description: We often take the Moon for granted. We assume it will always be there. Circling the Earth month after month with regular precision. But what would our world be like if we didn’t have a moon? The length of the day, the tilt of the planet, the number of meteors impacting the surface are just a few of the things that would all be vastly different if the Moon did not exist. Not to mention all of the familiar cultural references that would no longer make sense.

Bio: Patrick McQuillan earned a B.S. degree in Physics from the College of William and Mary. His senior research project involved determining the period of variable stars, most notably Alpha Auriga. This was at a time when collecting data meant going to the roof of the physics building, locating the research star by hand, and tracking the star manually by following a guide star in the finder scope. No GPS-auto-guiding-from-a-climate-controlled-remote-location! In the twenty plus years since then, he has explained astronomy to the general public as a Planetarium Director, the Education Manager for Challenger Center for Space Science Education, a NASA Solar System Ambassador, and currently explains Earth Science as Education and Outreach Specialist for IRIS. You can view current earthquake activity using the Seismic Monitor located on the IRIS website.

Today’s Sponsor: This episode of “365 Days of Astronomy” is sponsored by Astrocamp Summer Mission of Idyllwild, California. Help introduce a child to the world of Astronomy. Learn more at


Hello, I’m Patrick McQuillan, Education and Outreach Specialist with IRIS, the Incorporated Research Institutions for Seismology, a NASA Solar System Ambassador and a former Planetarium Director. Welcome to the March 28th edition of the 365 Days of Astronomy podcast: What If the Moon Did Not Exist?
One of my favorite comic books growing up was a Marvel comic entitled What If…? The comic usually began with a brief replay of a notable event from the story of superhero in Marvel Universe. At a critical story point, one which could have sent the story in several vastly different directions, the writers explored what would have happened if events had taken a different course. For example, what if Spiderman’s Aunt had died instead of his uncle? What if the Fantastic Four had not gained their super powers? What if Captain America lived during the American Civil War?

In almost every one of these stories the change led to something bad, or at least worse than the original story. Which of course led you to believe that the writers had made the best choice when they directed the paths of the original stories. And it led my friends and I to have long, heated arguments about the moral, social and philosophical implications of the various versions of the stories. Well… as much as you can argue about fictitious characters when you are twelve.

In science it can often be instructive to ask What If? Such thought experiments have been used for centuries to explore science concepts, usually when the actual experiment was too expensive or even impossible to actually perform.

So to take a page from Marvel comics, in today’s podcast we look back to the early days of the Solar System. We see the early Earth forming from the cloud of gas and dust surrounding the young Sun. An object, perhaps as large as Mars, is on a collision course with our future home. This giant collision destroys the incoming body and ejects large amounts of debris into orbit around the Earth. This cloud of debris eventually forms the body we know today as our Moon.

Now let us pause and pose our philosophical question…What if this collision NEVER HAPPENED?
We see the early Earth forming from the cloud of gas and dust surrounding the young Sun. An object, perhaps as large as Mars, is on a seeming collision course with our future home. They just miss. No collision. No cloud of ejected debris. No Moon. And I will give away the bad answer… No Earth as we know it!

Yes that is correct. Without the Moon, life on Earth would not be life as we know it.
The first thing that would be different are eclipses. They would be no more. No more solar eclipses, no more lunar eclipses. In fact there would be no more solar eclipses anywhere in the solar system. The earth is the only planet where, from the surface of the planet, one of the planet’s moons seems exactly the same size as the Sun and can just cover it, causing an eclipse.

There would be more consistently dark skies for astronomical observing if there was no moon. Astronomers would still have to compete with human created light pollution.

If we had no moon there would be no tides. Well not exactly. There would still be tides; they would just be smaller. Both the Sun and the Moon create tides on the Earth. There are two high tides and two low tides each day.

Tides occur because the gravitational force between two bodies decreases with distance. Gravity tugs on nearby objects more strongly than on distant objects. The oceans on the side of the Earth closest to the Moon feel the greatest attraction to the Moon (and to the Sun). Being fluid, the nearby oceans move upward in response to the Moon’s pull, until there is a balance between the upward force from the Moon and the downward force from the Earth. The high tide on the far side of the Earth occurs because the oceans on the side of the Earth farthest from the Moon feel less gravitational attraction to the Moon. They are left behind as the Moon pulls the other parts of the Earth toward itself with greater force.

This difference in gravitational attraction on the near and far sides of a body is officially known as tidal force. The word “tidal” does not refer to actual moving ocean tides, although this force does give rise to them. The strength of tidal forces fall off according to an inverse cube law. (Remember gravity falls off according to an inverse square law.) The inverse cube relationship means that tidal forces are more dependent on distance than gravity.

In simple terms, since the Moon is 400 times closer to us than the Sun, and even though the Sun is more massive than the Moon and thus has more gravity, the much closer distance of the Moon makes its tidal forces on the Earth much larger than the Sun’s. The tidal effect of the Sun is just under one half the tidal effect of the Moon.

With both the Sun and the Moon rising tides in the Earth’s oceans the actual height of high tide at any one location is complicated. First let us ignore the effects of local geography, which can cause really high tides due to the shape of the land into which the tide is moving. When the Sun, Moon and Earth are all lined up, the tides from the Sun reinforce those from the Moon and we have higher high tides. This is called a spring tide. It has nothing to do with the season of Spring. When the Sun, Moon and Earth are lined up in a 90-degree angle, the tides from the Sun partially cancel those from the Moon and we have lower high tides. This is known as a neap tide.

Thus, if we had no moon we would still have tides. They would be simpler. And they would not be as strong. So surfing would not be as exciting. Bummer.

But it gets worse. Due to the effects of tides, the Moon is gaining angular momentum from the Earth. The Moon’s orbital velocity is increasing and it is moving away from the Earth. Due to conservation of momentum, the Earth’s period of rotation is slowing down. The day is getting longer. The day increases by 0.002 seconds each century. Which doesn’t seem like much, but over billions of years it adds up.
This exchange of energy occurs mostly because the eastward rotating Earth is rubbing against the westward moving tides. This causes friction, which slows the Earth’s rotation. Since the total energy of the Earth-Moon system must remain constant (conservation of Momentum), the Moon’s velocity increases. As a body’s orbital velocity increases, it must move farther away from the object it is orbiting in order to remain in orbit. The Moon is receding from the Earth about 2 inches per year. This has been confirmed by bouncing laser beams off reflecting mirrors left behind on the lunar surface by the Apollo astronauts and measuring the round trip travel time of the laser light.

If the Earth is slowing down, it must have been rotating more rapidly in the past. By counting growth rings in 400 million year old coral fossils and in 3 billion year old stromatolites, geologists calculate that the Earth was rotating four times faster when it formed than it is today. The tidal effects of the Moon and, to a much lesser degree, the Sun have lengthened the day from six hours to 24 hours.

By the way, the closest the Moon could have been was about 7,300 miles above the Earth’s surface. This is 1/20th the Moon’s present distance. Any closer the tidal forces created on the Moon by the gravity of the Earth would have ripped the Moon apart, turning it into a ring. This limit on the Moon’s distance is consistent with the theory of lunar formation by giant collision.

Ok, so with no Moon to slow the Earth down, the only tides would be from the Sun. The Earth’s rotation would still slow down, by not nearly as much. The six-hour day at the time the Moon should have formed would increase to an eight-hour day at present time with no Moon. Not good for life on Earth!

The faster a planet rotates, the faster its winds blow. Just look at Jupiter. The largest planet in the solar system rotates once every 10 hours. Winds are pulled into east-west flowing patterns with wind speeds between 100 and 200 miles per hour. The Earth would be rotating once every eight hours, so 100 mile per hour wind speeds would not be unreasonable to expect. Hurricanes would have even higher wind speeds.
Try to imagine what life would be like with only three to four hours of sunlight each day. Life forms on Earth have evolved with biological clocks set to the familiar 24 hours of rotation. Many activities are regulated by internal biological clocks. Waking, sleeping, hunger, and mating depend on these circadian rhythms. Animals with a 24-hour internal clock would quickly get out of sync with an eight-hour day. Think about how you feel after an airplane trip that crosses multiple time zones.

Of course life would have evolved to fit the conditions of a shorter day. It just might not have included humans. Which I guess would be ok, there are so many things I don’t get done during a 24 hour day, I would not even like to speculate how little could be accomplished in an 8 hour day.

Let’s not forget that the mere presence of the Moon caused less life ending impacts to occur on the Earth. Without the Moon’s helpful goal keeping, the Earth would have experienced a higher rate of impact from space debris as the solar system, and life, was forming. Which might have led to the absence of humans on Earth today.

And even if humans did exist on this wildly rotating, wind tunnel of a planet, think of all the cultural references to the Moon that would not exist.

The word “month” would not exist. With no Moon as a reference, there could be no lunar calendar. Time keeping would be more difficult. The Sun would be the next best reference point.

There would be no “lunatics”. Well, they would still exist; we would just need another word for them. Nightatics just doesn’t seem to cover it.

There would be no lunar fables. Gone would be werewolves, humans who change into the form of a wolf during full moons.

With no moon to travel to, our first foray off the Earth to a distant world would not have occurred. Mars would be the nearest destination. A trip to Mars is a much more complicated three year journey compared to the, although difficult, but much shorter, two week trip to the Moon.

For those with romantic inclinations, there would be no full moon to park beneath. Somehow, I think a night under a bright Venus would not elicit the same response.

And think of all the wonderful songs that would either not exist or would have been written differently. Familiar favorites such as: Nothing River, Blue Nothing, Fly Me to the Nothing, Nothingshadow, Blue Nothing of Kentucky, Nothing over Bourbon Street, Bad Nothing Rising, By the Light of Silvery Nothing, Nothingdance, and my personal favorite: Pink Floyd’s Dark Side of the Nothing.

So while we often forget it is around, we should take time out every now and again to reflect on how different the Earth would be without our nearest neighbor. And if you take along a close friend the contemplation could be doubly enjoyable.

365 Days of Astronomy
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