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I am trying to build a full-scale model of the solar system.

In an empty scene I did this:

  • Add a 6378m radius sphere called "Earth"
  • In View set Clip Start and End to 1000m and 150000000000m (1AU)
  • Set the light x coordinate to 150000000000 and power to 10000000000000000000000000W
  • Set the camera location to (0,6380,0) and angle to (90,0,0), this puts it on the surface of Earth, looking outwards on the Y axis. Apply all transformations.
  • Put the Earth and Camera into an "Earth System" empty.
  • Created a 1740m radius sphere called Moon. Set its Y coordinate to 20000m and put it in an Empty called "Moon Orbit"
  • Set Camera Clip Start and End to 1000m and 150000000000m

At this point if I view through the camera, the moon is unrealistically close but it does render

enter image description here

If I set the moon Y coordinate to 385000m then it looks smaller, unsurprisingly.

enter image description here

But it's already too small. If I put in a realistic lunar distance of 385000000m then it vanishes completely.

I think my numbers are physically accurate, so why is the moon vanishingly small when rendered in the camera?

(The attached blend has the Moon Y at 385000m which is 1000 times too near, but is visible)

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    $\begingroup$ Why do you have to place the moon so far away? You don't need a realistic scale just to show the solar system, I have no idea why you would work with numbers that Blender can't handle, use the smaller ones as if you were building a small copy of it. $\endgroup$
    – Hatrutsu -FKA KiskaArt-
    Commented Mar 16 at 16:01
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    $\begingroup$ @KISKAart It is already scaled down by a factor of 1,000, although the OP did not seem to realize it and did it accidentally. But I agree with you, why go full scale? I would maybe even scale down to 1:1,000,000 $\endgroup$
    – Gordon Brinkmann
    Commented Mar 16 at 16:16
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    $\begingroup$ @spraff Just by the way, where did you get the value for the power of the light? If I counted correctly you have $10^{25}W$, the sun has a power of $3.828\cdot 10^{26}W$. The power reaching the Earth's surface is usually given in $W/m^2$, which is the unit the Sun lamp in Blender uses. That would be $1361\, W/m^2$. But unfortunately the Sun lamp is only a directional light (which I usually ignore when just using it for Earth and Moon because then a Track to constraint on the light makes it always point towards Earth). $\endgroup$
    – Gordon Brinkmann
    Commented Mar 16 at 20:44
  • $\begingroup$ ...but another thing is, our Sun is not a point light, it emits this power in total over the complete surface. $\endgroup$
    – Gordon Brinkmann
    Commented Mar 17 at 8:22

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You have set the Moon to a radius of 1,740 m as you said. Its radius is 1,740 km = 1,740,000 m. You have also placed the camera at 6,380 m, while the surface of the Earth should be 6,380 km = 6,380,000 m.

So you have scaled everything down to 1:1,000, in this case the distance of the Moon is 385,000 m, not 385,000,000 m. Which is the distance you have said to show in your last image.

If you did not change the default focal length of your camera, it is 50 mm, which gives a horizontal angle of view of 39.6°, the Moon has an angular size of approx. 0.5°, making it about 24 pixels wide in a 1920 × 1080 pixels image taken by a 50 mm lens.

Here is your image with the small Moon, scaled to 1920 × 1080, and above it is a circle of 24 pixels in diameter. Looks quite identical and therefore correct to me.

moon size

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