<p>In general this is not true.  Most celestial objects appear as points of light to the naked eye, and don't have any apparent size. (Though your brain might conflate &quot;bright&quot; with &quot;near&quot; or &quot;large&quot; - giving rise to ufo reports about Venus)</p>
<p>Objects like the moon have a clear edge and making the moon brighter wouldn't make it larger.  There are a number of optical illusions relating to the moon's size, but generally the optical illusion is that it seems larger when it is close to the horizon, and so actually dimmer (due to absorbsion of light by the atmosphere).</p>
<p>But some objects have no clear edge.  Objects like galaxies just get fainter and fainter as you get further from the centre. A telescope that can see fainter light would see a larger body than one that can only see the central, brightest part of the galaxy.  Comets, likewise, can appear larger if they are brighter, simply because we can see the generally fainter outer parts of the comet if it is brighter.</p>
<p>And the optical limitations of lenses and mirrors will turn a point source of light into a (very) little fuzzy disc.  This is called the &quot;point spread function&quot;.  As stars appear as little fuzzy discs, without sharp edges, brighter stars will appear as bigger in photographs taken of the night sky.</p>

CC BY-SA 4.0
2023-12-12T14:22:14.327Z
+1, "But some objects have no clear edge..." Case in point: [How could I see Andromeda as large as in this picture?](https://astronomy.stackexchange.com/q/49370/7982) and [Is this photocomposition of Andromeda's galaxy and the Moon accurate?](https://astronomy.stackexchange.com/q/23378/7982) and [Is the scaling in this composite photo of the moon and the Andromeda galaxy reasonably correct?](https://astronomy.stackexchange.com/q/49367/7982) and [How big is the Andromeda galaxy in the night sky?](https://astronomy.stackexchange.com/q/24184/7982)
uhoh
CC BY-SA 4.0
2023-12-12T14:22:18.937Z
and [Apparent size of M31](https://astronomy.stackexchange.com/q/1025/7982)
uhoh
CC BY-SA 4.0
2023-12-12T07:36:34.403Z
2023-12-12T07:36:34.403Z
James K
<p>Is it true that given two celestial objects of the same size and at the same distance, the fainter one will appear smaller? Is there a calculation that can be made about how smaller? Is it the same for naked eye observation or telescope observation?</p>

CC BY-SA 4.0
2023-12-11T19:21:14.213Z
No, this is not, in general true.  Most celestial objects do not appear to have any size at all and only appear as points of light.  Perhaps you are talking about an optical illusion or misconception that brighter stars are closer.
James K
CC BY-SA 4.0
2023-12-11T19:23:05.253Z
But if two object have the same size and the same distance, then they will have the same angular size,
James K
CC BY-SA 4.0
2023-12-11T19:37:15.753Z
@JamesK I might be thinking something wrong, but consider a celestial object (star/planet). The edges are not well defined but rather smoothed due to resolution. The intensity cutoff (which depends on the sensor, your eyes, a camera, etc) will determine how big the actual object appears to be. It's like having a gaussian with threshold in the altitude: the percieved width seems to change, right?
Kim
CC BY-SA 4.0
2023-12-11T19:46:00.527Z
Perhaps that is an effect with object that actually have indistinct edges, like galaxies.  But not with planets and the like, Their edges are very clear and sharp.
James K
CC BY-SA 4.0
2023-12-11T21:06:20.963Z
It sounds like you might be talking about the "point spread function", which is an optical effect that causes brighter pinpoints of light to appear larger. https://en.wikipedia.org/wiki/Point_spread_function
Greg Miller
<magnitude><angular-resolution><angular-diameter><brightness>
Visible angular size
Kim
# Visible angular size

(Asked by: username_0 on 2023-12-11)

Is it true that given two celestial objects of the same size and at the same distance, the fainter one will appear smaller? Is there a calculation that can be made about how smaller? Is it the same for naked eye observation or telescope observation?

**Tags:** magnitude, angular-resolution, angular-diameter, brightness

> **Comments:**
> **username_1**: No, this is not, in general true.  Most celestial objects do not appear to have any size at all and only appear as points of light.  Perhaps you are talking about an optical illusion or misconception that brighter stars are closer.  
> **username_1**: But if two object have the same size and the same distance, then they will have the same angular size,  
> **username_0**: @JamesK I might be thinking something wrong, but consider a celestial object (star/planet). The edges are not well defined but rather smoothed due to resolution. The intensity cutoff (which depends on the sensor, your eyes, a camera, etc) will determine how big the actual object appears to be. It's like having a gaussian with threshold in the altitude: the percieved width seems to change, right?  
> **username_1**: Perhaps that is an effect with object that actually have indistinct edges, like galaxies.  But not with planets and the like, Their edges are very clear and sharp.  
> **username_2**: It sounds like you might be talking about the "point spread function", which is an optical effect that causes brighter pinpoints of light to appear larger. https://en.wikipedia.org/wiki/Point_spread_function  

---

## 1 answers

### Answer by username_1 on 2023-12-12. Score: 2

In general this is not true. Most celestial objects appear as points of light to the naked eye, and don't have any apparent size. (Though your brain might conflate "bright" with "near" or "large" - giving rise to ufo reports about Venus)

Objects like the moon have a clear edge and making the moon brighter wouldn't make it larger. There are a number of optical illusions relating to the moon's size, but generally the optical illusion is that it seems larger when it is close to the horizon, and so actually dimmer (due to absorbsion of light by the atmosphere).

But some objects have no clear edge. Objects like galaxies just get fainter and fainter as you get further from the centre. A telescope that can see fainter light would see a larger body than one that can only see the central, brightest part of the galaxy. Comets, likewise, can appear larger if they are brighter, simply because we can see the generally fainter outer parts of the comet if it is brighter.

And the optical limitations of lenses and mirrors will turn a point source of light into a (very) little fuzzy disc. This is called the "point spread function". As stars appear as little fuzzy discs, without sharp edges, brighter stars will appear as bigger in photographs taken of the night sky.

> **Comments:**
> **username_3**: +1, "But some objects have no clear edge..." Case in point: [How could I see Andromeda as large as in this picture?](https://astronomy.stackexchange.com/q/49370/7982) and [Is this photocomposition of Andromeda's galaxy and the Moon accurate?](https://astronomy.stackexchange.com/q/23378/7982) and [Is the scaling in this composite photo of the moon and the Andromeda galaxy reasonably correct?](https://astronomy.stackexchange.com/q/49367/7982) and [How big is the Andromeda galaxy in the night sky?](https://astronomy.stackexchange.com/q/24184/7982)  
> **username_3**: and [Apparent size of M31](https://astronomy.stackexchange.com/q/1025/7982)  

---

