How the Earth gave the Moon a lemon shape

My annual birthday-month fund-raising drive for Behind the Black is now on-going. Not only do your donations help pay my bills, they give me the freedom to speak honestly about science and culture, instead of being forced to write it as others demand.


Please consider donating by giving either a one-time contribution or a regular subscription, as outlined in the tip jar below.


Regular readers can support Behind The Black with a contribution via paypal:

Or with a subscription with regular donations from your Paypal or credit card account:

If Paypal doesn't work for you, you can support Behind The Black directly by sending your donation by check, payable to Robert Zimmerman, to
Behind The Black
c/o Robert Zimmerman
P.O.Box 1262
Cortaro, AZ 85652


You can also support me by buying one of my books, as noted in the boxes interspersed throughout the webpage. And if you buy the books through the ebookit links, I get a larger cut and I get it sooner.

Scientists have found that the Earth’s gravity combined with the Moon’s rotation forced the satellite to become “lemon-shaped.”

As the Moon solidified, its rotation caused it to elongate along its polar axis. But because the length of the Moon’s rotation was the same as its orbit, with one hemisphere always facing the Earth, the tidal force of the Earth’s gravity then pulled at the center, distorting the Moon’s shape so that one hemisphere bulged Earthward.

This theory is not new, but these new calculations are more robust, lending greater weight to it.


One comment

  • mpthompson

    I’ve always wondered if the maria which dominate the near side of the moon were essentially caused by tidal action on the young Moon whereby the Earth’s gravity literally pulled an ocean of liquid magma up through fissures on the young thin crust. This magma would have pooled and formed the maria floors. Given the Moon was much closer to the Earth during its formation, Earth’s gravity was probably one of the dominating factors that give the Moon a number of odd characteristics.

    I’ve read other theories regarding the maria that internal radioactivity that kept the Moon’s interior on the near side hotter than on the far side, but I’m don’t see a mechanism by which radioactive elements wouldn’t generally be evenly distributed within the Moon’s interior. Or other theories that the Earth somehow steered large objects to predominantly strike the near side of the moon, but this doesn’t seem to make much sense either. Finally, I’ve seen other theories that the young Earth’s surface was so hot that it heated up the near surface of the Moon and filled the maria with magma from this heat, but I don’ think the Moon was ever close enough for heat that radiated from Earth to have much impact on the moon.

    Anyway, fun stuff to think about.

Leave a Reply

Your email address will not be published. Required fields are marked *