February 13: A scale model of the Earth-Moon system


365DaysDate: February 13, 2009


Title: A scale model of the Earth-Moon system

Podcaster: Bill Hudson

Organization: Astrogeek  www.astrogeek.wordpress.com

Description: How to make and use a scale model of the Earth-Moon system to emphasize the large distances to astronomical objects.  The audience for this podcast are school teachers or amateur astronomers who are involved in public outreach programs. This podcast will describe how to make the scale model (1cm = 1000km) using simple materials (cardstock, a drawing compass, a ruler, and a pair of scissors) and how to use the model in an interactive activity with an audience.  This activity will visually demonstrate the scale of the Earth-Moon system and emphasize the distance to the Moon.

Bio: Bill Hudson is an amateur astronomer, and a professional computer geek.   He can frequently be found at Fremont Peak Observatory, helping kids up the observation ladder of the 30″ Challenger telescope.  He is proud to be passing on his love of astronomy to his children, two of which have been certified to operate the Challenger.

Today’s Sponsor: This episode sponsored by John Rummel and dedicated to his much loved and ever patient wife, Cathy, who tolerates all manner of obsessions with astronomy, science, photography, football, and other expressions of general geekiness.


Hi everybody, this is Bill Hudson, and I’m an amateur astronomer.

I’m one of the volunteers at Fremont Peak Observatory, which is an amateur-run observatory located inside Fremont Peak State Park, which is just South of San Juan Bautista, California.  Our observatory is open to the public for scheduled viewing and educational programs from April through October on Saturday evenings that don’t include a full moon, which is usually three Saturdays each month, and they can be found online at http://www.fpoa.net

The theme music for this podcast is very appropriate to this episode, because I want to talk about how to communicate some of the distances and sizes of astronomical objects.

I’ve been an amateur astronomer for about ten years now, and I’ve been giving presentations to local fifth-grade classrooms for about three years.  Astronomy is taught at the fifth-grade in California, so you’re talking to ten and eleven year old students who still think Astronomy is pretty cool.  One of the catches of doing presentations to elementary school students is that you really need visual aids, because otherwise you’re just up there, talking by yourself, which is kind of scary for you, and kind of boring for them.

When I got started doing presentations, I really wanted a scale model of the Earth and Moon, and I couldn’t really find anything that was an appropriate size, so I decided to make one.  What I did is very simple, I took a drawing compass, a metric ruler, and a piece of cardstock, and a pair of scissors.  I drew circles of what I thought was the appropriate size, and I labeled one “Earth”, and I labeled the other “Moon”, and off I went.  Turned out that they were way too big as well.

After some initial embarrassment and experimentation, I finally settled on a scale of one centimeter for a thousand kilometers.  It turns out this is a very good scale, because the Earth and Moon wind up being about twelve and a half feet apart, which is an appropriate size for a stage.

So at this scale, the Earth is going to be 12.8 centimeters in diameter, and the Moon is going to be 3.5 centimeters in diameter.  If you’re artistic, you might want to draw on them a little bit, make them look like the Earth and Moon.   I tried that once, and the results were so bad my daughter laughed at me, so I have her do it from now on.

So, now you’re standing up on stage, you’ve got two cardboard circles in your hands, and a hundred fifth-grade students staring at you.  What do you do?

Using these requires a little bit of advance preparation.   You have to mark off 384 centimeters.  Now if you don’t have a long metric tape measure, which I don’t, that is 12 feet, 7 and 3/8 inches.   I usually mark these off with something inconspicuous, something like a small pencil mark, a post-it note, or even pennies, something that the students who are sitting in the audience can’t see.

Usually I’ll introduce the subject of distance by talking about something that they can relate to, which is riding in a car.    Usually what I’ll do is I’ll ask who’s driven to San Francisco.  San Francisco is about two hours away from my home.  I then relate that speed to time, and talk about how long it would take to drive to a more distant location.  Now sometimes I’ll pick well known destinations like Las Vegas, or Denver, or Chicago.  When I talk about the size of the Earth, I talk about how long it would take to drive all the way around it, and to make things easy – for me, I mean – I tell them that 60 miles per hour is almost 100 kilometers per hour, and that since we’re being scientists that day, we’re going to use that value.  I tell them the circumference of the Earth (about 40,000 kilometers) and I ask them to figure out how many hours that would be.  Depending on the size of the group I may ask one of them to make the conversion, or I may just give them the answer, which is almost 17 days.

Then I give them the value of the Earth’s diameter, and I’ll hand the Earth cut-out to one student, or for a large group I may have a teacher do this part, and I’ll have them stand on one of the pre-marked spots.   I give the Moon cut-out to a second helper, and I have the group guess how far away they should stand from the Earth to make it the right distance.  I’ll ask the group to vote for “It’s too close” or “It’s too far” or “It’s about right” until I get about the same number of kids in the ‘too close’ and ‘too far’ groups.  Usually… not always, usually they’ll guess about half to two-thirds of the actual distance.  At this point you can introduce some of the larger distances like the Earth-Sun distance.   The beauty of the scale of one centimeter to a thousand kilometers is that one astronomical unit is almost exactly one mile.   It works out to be just over nine-tenths of a mile, so I’ll frequently prepare for my presentations by selecting a local building or landmark that is about a mile away, and you can also talk about some of the larger objects in the solar system, like the Sun, which would be about 14 meters in diameter, or Jupiter which would be 1.4 meters in diameter.

I hope that my talking about this has given some of you some ideas on how to get started in communicating astronomy to the public, and maybe making an effort this year, the International Year of Astronomy, to go out to your local school district, and volunteer your efforts as an amateur astronomer and be a volunteer interpreter, maybe take a solar telescope out.  The teachers really, really appreciate it, and the kids enjoy it as well.

Thank you very much for listening.

A PDF scale drawing is available at http://astrogeek.wordpress.com/resources/

365 Days of Astronomy
The 365 Days of Astronomy Podcast is produced by the New Media Working Group of the International Year of Astronomy 2009. Audio post-production by Preston Gibson. Bandwidth donated by libsyn.com and wizzard media. Web design by Clockwork Active Media Systems. You may reproduce and distribute this audio for non-commercial purposes. Please consider supporting the podcast with a few dollars (or Euros!). Visit us on the web at 365DaysOfAstronomy.org or email us at info@365DaysOfAstronomy.org. Until tomorrow…goodbye.

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5 Responses to February 13: A scale model of the Earth-Moon system

  1. Idle Astronomer February 13, 2009 at 8:32 am #

    This is the podcast from 12 Feb repeated. Please can you find the time to correct it and repost the program.
    (Friday the 13th strikes!)
    Keep up the other wise good work.

  2. Gavino February 13, 2009 at 11:07 am #

    This is the incorrect podcast. It’s the same as the Feb 12th one.

  3. michael February 13, 2009 at 11:16 am #

    Sorry! We are working on it…

  4. Richard Drumm February 16, 2009 at 9:29 am #

    I’m not sure how iTunes will deal with this one if it has the same title. It might not notice it at all. Perhaps the same title with “repost” added to the end.
    Thanks, Michael!
    Richard Drumm The Astronomy Bum

  5. Edward J. Turk July 3, 2009 at 4:57 pm #


    What is the RA of the forthcoming Moon’s ascending node and descending node?

    I want to know the date and hour when this occurs in Cleveland Ohio and then anywhere else on the planet, if this is possible without some horrendous celestial mechanics calculations.

    I say this because of ther 18,6year cycle of the line of nodes. I have no idea where in this cycle the line currently is. Everytime I seek this answer, I do not even get a reply in any form.
    Tx for any help.

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