# How heavy is a 30-inch globe?

I’d like a hollow plaster globe. Not a solid one, which would weigh 433 pounds, but a hollow one.

So how thick should we make the plaster? Initially, I’d guess about an inch thick as a minimum, with some internal reinforcement possibly, to prevent cave-in’s. So, just how heavy would this globe be? Let’s work out the weight…

If we want a 1-inch thick layer of plaster, the volume would be equal to a volume of a 30-inch sphere minus the volume of a 28-inch sphere. The volume for a sphere is calculated as V=(4/3) * pi * r3. Therefore, if a 30-inch sphere is 14137.166943 cubic inches and a 28-inch sphere is 11494.0403234 cubic inches, then the volume of a 1-inch thick layer of plaster would be 2643.1266196 cubic inches (14137.166943 – 11494.0403234 = 2643.1266196).

The density of plaster is equal to 0.490753134528 ounces per cubic inches, and the formula for weight is w=p*v. Given p and v, we can calculate the weight of the globe as 81.07 pounds.

A little heavy, but workable with a strong base.

How about some other thicknesses? Using the same formulas, we can work out some other weights for a 30-inch globe based on the thickness of the plaster:

Thickness (inches) Volume (cubic inches) Weight (pounds)
0.5 14137.166943 – 12770.0505397 = 1367.1164033 41.93
1.0 14137.166943 – 11494.0403234 = 2643.1266196 81.07
1.5 14137.166943 – 10305.9947014 = 3831.1722416 117.51
2.0 14137.166943 – 9202.77208112 = 4934.3948618 151.35

I think that a 1-inch layer of plaster is just about right.

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### 2 Responses to How heavy is a 30-inch globe?

1. Bill Thoen says:

Hi Mark,
I’ve been looking for information on the construction of globes (for years) and found it only relating to the party toys (blow up a balloon and cover it with papier mache, let it dry then paint the land green and the oceans blue (even thought we know from the Greeks that color of the sea is “wine-dark”) and after the paint dries…. you’ve got a great model of an irregularly shaped parody of Earth that look more like Mars’ moon Phobos. It’s a fun project for kids, but it isn’t what I’m looking for.

When I saw your descriptions of the process of building high-quality, 30″ globe, I said to myself, “That’s it! Just what I have been looking for!” So I have perused your web site looking for any topics on the construction process and techniques. There’s two things I’d really really like to know more about. What does the steel frame that supports everything look like and how is it put together, and how to apply the plaster and get it so perfectly smooth and round. (I can see it in the photo under the gore you were trying on for size.) That base looks pretty good! How did you do it?

If there’s any chance you could show more details of the frame construction, or its parts and how the spindle is anchored to the frame, that would be outstanding. Also, if you could recommend any books or URLs on the subject, or have a spare moment to point me in the right direction I would be much obliged.