World Builders™                                                                       Session One  --  Astronomy                            
                      Comparing

                                Apparent Moon Sizes

                       
           

How big is the moon?  How far away is it?  


You should read
Can You See Your Moon before you read this page.

When you see a moon in the sky, what you see is affected by

    • the actual size of the moon
    • how far away the moon is

Just as on earth, a big object very far away looks small. A small, distant moon may look like a dot of light, like a star.

Here is a table that Dr Viau has made for you to help you to understand what we are getting at here.

We see that when the Tangent of Theta has more than 3 zeros right after the decimal, we get a dot of light in the sky. You may see the dot moving, like a planet, but you will not get exciting moon effects with this satellite.

 Earth to  Distance
from Sun km

 Distance from Earth km

 Diameter km

 Diameter
Distance

 Tangent of Theta

 Viewing
Angle

 We
see

 Moon  

 384,400

 3476

3476 
384,400

 0.009

 0.5

 Disk

 Mercury    57,910,000

 91,690,000

 4880

    4880   
91,690,000

 0.00005

 0.003

 dot

 Venus   108,200,000

 41,400,000

 12,104

 12,104
41,400,000

 0.00029

 approx.
0.02

 bright
dot

 Mars   227,940,000

 78,340,000

 6794

  6794
78,340,000

0.0001

 

 dot

 Jupiter   778,330,000

 628,730,000

 142,984

 142,984
628,730,000

 0.00023

 approx 0.02

 bright
dot

 Saturn 1,429,400,000

 1,279,800,000

 120,536

 120,536
1,279,800,000

 0.00009

 

 dot

 Uranus 2,870,990,000

 2,721,390,000

 51,118

 51,118
2,721,390,000

 0.00002

 

 dot

 The Sun  

 149,600,000

 1,390,000

 1,390,000
149,600,000

 .00929

approx. 0.5

 Disk

Click here for the Tangent Table



We see that on earth the moon and the sun appear to be about the same size even though the sun is actually much, much bigger. However, the sun in also much farther away.  During a solar eclipse, the moon just about covers the sun, as in this picture.

How can you get the biggest possible moon in your sky?

You will need to study orbits and The Roche Limit to find out!

Then check out this page about Big Moons!

 Here is something else to think about.

This is a diagram of our moon, which is 0.5 degrees of viewing angle across.


Compares sizes of moon and smaller bodyWhat happens if we make a moon that has a viewing angle only half the size of our moon?

This moon would look as if it had half the diameter of our moon.

You can see that half the diameter is not the same as half the moon!

 Actually, you only get a quarter of the area (disk surface).

 Your moon is going to look quite a bit smaller in the sky. 

                                        It will not give as much light.


Compares moon to moons with one fifth the diameter.
          Here I cut the viewing angle into fifths!  
   
          Look at the tiny little moons I got!    

          Each one has only one 25th the area of our moon.

Would these little moons look any different from bright stars?


What About the Albedo?
The albedo measures how much light an object reflects.

Example: a mirror reflects nearly all the light that falls on it: the albedo of a mirror is perhaps 0.96, which is nearly one.

Example: a black sweater, which is dark and rough, reflects very little light. Its albedo might be 0.05.

The numbers that describe albedo go from 0 (no light reflected)to 1 (all light reflected).

Remember that planets and moons shine by reflected light. A planet or moon with high albedo will shine brightly, but it will not look any larger than it would if its albedo were low.

These two moons are the same size and the same distance from a star.
They both receive the same amount of light.

 This is a high albedo moon.

 This is a low albedo moon.

 A moon or planet with high albedo:

  • shines brightly
  • can be seen for a long distance

 A moon or planet with low albedo

  • does not reflect much light
  • can be seen close up but soon gets dim

Example: Venus has an albedo of 0.7 (This means it sends back about 70% of the light that falls on it.) It is the brightest planet, and easily seen in the sky. However, its viewing angle is small, and it looks like a bright dot in the sky.

Example: Our moon has an albedo of only 0.02, which is about 2% of all the light that it receives from the sun. The moon seems bright to us because we are close to it.  


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© 2000, 2003. Elizabeth Anne Viau. All rights reserved. This material may be used by individuals for instructional purposes but not sold. Please inform the author if you use it at eviau@earthlink.net.