Why is the moon so small in Stellarium?

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Larry G
2011-02-02
2013-05-01

• Larry G
2011-02-02

I have a question about the moon in Stellarium. Now admittedly I am no
astronomer, and I haven't had any kind of astronomy class since grade school,
lol. But why is the moon so small in Stellarium? I know there is something
called "the moon illusion" and it appears larger at the horizon due to
atmospheric refraction or something, but I KNOW that the moon is big enough I
should be able to see the shadow when I look in the actual sky. I know if I
scale the moon (x4) I can see the shadow and such, but I find if I scale it to
(x8) it seems to be the size we see it in the sky, but yet according to
Stellarium, from my location, the moon is just off the upper right of the sun
and if it were that big I'd probably see it, lol. So what is happening there?
lol Any astronomers? lol

• Bogdan Marinov
2011-02-02

The default field of view of Stellarium is 60 arc-degrees of the sky. (Look at
the FOV value in the bottom). At the same time, it is "squeezed" into a screen
that takes less than 60 arc-degrees of a person's vision.

At the same time, if an object's angular diameter drops below a certain value,
Stellarium replaces the 3D-rendering with a standard "star" image that depends
on the object's brightness.

So, depending on the FOV/zoom, the Moon may appear smaller/larger than what
you would see if you looked at the sky through a window with the size of your
screen.

• larryg333,

The Moon, to our eye, does not change size. It is always about 0.5 degrees in
size. If you hold your arm outstretche and then hold your pinkie finger under
the Moon, you'll see it is about the size of your fingernail. Do that when
it's low or high in the sky. It'll be the same. Do that at perigee (closer to
Earth) or apogee (further away) and it will look exactly the same.

The "moon illusion" is your brain fooling you. It looks bigger when near
objects and structures and trees on the ground. It looks small in wide open
sky or outer space. Your brain is messing with you. Prove it with the pinkie
fingernail size test.

Stellarium can show the Moon in its exact real size. And if you zoom in really
tight so it fills the whole monitor and speed up time, while tracking, you'll
see the Moon swell as it draws close at perigee and shrinks at apogee. Zoom
out and you'd be hard-pressed to see any difference. Just like the real thing.

All that said, Stellarium has a switch setting called Scale Moon. Go into the
Sky and view options window and click the Sky tab if nec. In the
Planets and satellites group, note the Scale Moon checkbox. Checked, the
Moon will be much larger, 4x larger, 2 degrees in size. Artificial, yes; but
it helps you spot it in day time or a busy part of the night sky.

Hope that helps.

cheers,
Blake

• martin harris
2011-02-11

The moon is an imortant part of my sky viewing, as it tells me how bright it
will be on any given night. For example the moon is just about to move into
Orion and will be bright at this time so the software is telling me. It would
be neat to have a tool that can show the relative brightness on any particual
night without having to follow the moon. Jupiter is bright right now, but the
moon is so close its hard to get a good look at it. When would be a good time
to look at Jupiter? how much will the moon interfere with viewing on a night?

• barrykgerdes
2011-02-12

Hi dewar34

Stellarium does just that. The relative sky brightness does in fact follow the
relative glow of the moon. Select a view in the late evening (After the sun
glow has faded) when the moon is new and advance the clock by 1 day at a time
so the moon will go through a full cycle and watch the effect as the moon
follows its path. The darkening of the sky will depend on the phase of the
moon.

Barry

• Anonymous
2013-04-30

Not only does it show the sky glow according to the moon crescent and position in the sky, but it also has a beautiful 'Dynamic Eye Adaptation' feature right next to the Scale Moon checkbox, which automatically suppresses the dimmer stars and other objects in the vicinity of the moon, simulating the effect of our eye pupils contracting in response to the brightness of the moon. We're talking about two separate causes here - one, the sky glow due to the moon's presence and two, the eye's response to the moon's brightness, and Stellarium simulates both these variables.

Anonymous