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69 votes
Accepted

Where in space would the Earth and Moon appear to be the same size?

When viewing a sphere of radius $r$ at a distance $d$ from the centre of the sphere, you don't see a circle of radius $r$. The extreme lines of sight are tangents to the sphere, as this diagram ...
PM 2Ring's user avatar
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24 votes
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Approximately what percent of the sky has nothing in it?

It is really quite hard to answer the question as posed because as you observe deeper and deeper (e.g. using a larger telescope or observing for longer) then more and more (fainter) objects become ...
ProfRob's user avatar
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23 votes
Accepted

Can the Earth be seen from the moon when we have a total lunar eclipse at the perigee?

The Sun eclipsed by the Earth was actually recorded in 2009 by the Japanese lunar satellite Kaguya (athough this was strictly speaking not a total eclipse as such; see explanations below) Here the ...
Thomas's user avatar
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18 votes
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What would Betelgeuse look like from Earth if it was at the edge of the Solar System

The distance to Betelegeuse is not precisely known for reasons you can read about here and here. But let's assume a likely distance of 200 pc. The angular diameter of the star has been measured with ...
ProfRob's user avatar
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17 votes
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Understanding The Turnover Point of Angular Diameter Distance

On the one hand an object spans a smaller angle the farther away it is, as expected. On the other hand, due to the expansion of the Universe and the finite speed of light, very distant objects were ...
pela's user avatar
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17 votes

Approximately what percent of the sky has nothing in it?

Zero. At the limit, there's the Cosmic Microwave Background which represents the content of the Universe not long after it came into being. Of course, some would no doubt propose that since the CMB ...
Mark Morgan Lloyd's user avatar
16 votes

Can the Earth be seen from the moon when we have a total lunar eclipse at the perigee?

Can the Earth be seen from the moon when we have a total lunar eclipse at the perigee? Yes - but primarily it's the Earth's atmosphere that you would see. Earth's atmosphere will refract and scatter ...
uhoh's user avatar
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14 votes
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Why does the angular size of the Moon change?

The moon's orbit is elliptical, and it moves faster when it is closest to the Earth. This is the case for all elliptical orbits, and was discovered by Kepler, who gave a neat rule for predicting how ...
James K's user avatar
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12 votes
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What is the furthest star for which we can measure the diameter?

It's neither the angular diameter or prallax precision that is the limiting factor, but the fact that it is difficult to get the interferometric measurements for faint stars. State-of-the-art ...
ProfRob's user avatar
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11 votes

Where in space would the Earth and Moon appear to be the same size?

The Earth's radius is about 6371 km, while the moon is 1737.4 km. The distance between the two bodies is 384399 km. The formula for angular size (in radians) is $\delta = c$. Letting $x$ be equal to ...
WarpPrime's user avatar
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8 votes

Can the Earth be seen from the moon when we have a total lunar eclipse at the perigee?

The Moon as seen from the Earth looks dark red during a total lunar eclipse — it doesn’t go completely black and disappear. That’s because it is still partly illuminated by sunlight refracted through ...
Mike Scott's user avatar
8 votes

Approximately what percent of the sky has nothing in it?

Following on @mlk's suggestion to do Olbers' paradox in reverse, I'll try to estimate how much of the sky ends at a visible star (i.e., emitting something in the visible range, so that it could ...
Michael Seifert's user avatar
8 votes

About angular diameter, parallax and image of the nearest neutron star RX J185635-3754

With very few exceptions of the biggest and most nearby sources (like Betelgeuze with <~50mas) we cannot resolve the angular diameter of stellar-type sources by direct imaging means. Precision of ...
planetmaker's user avatar
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7 votes

About angular diameter, parallax and image of the nearest neutron star RX J185635-3754

All images of stars (bar one or two of the closest or largest) are effectively those of point sources. The image we see is the convolution of a point with the instumental "point spread function&...
ProfRob's user avatar
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6 votes
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Should the synodic period or the orbital period be used to determine the diameter of the Moon?

What you need, as Aristarchus, is a frame of reference in which the Earth's shadow is stationary. Otherwise you are trying to deal with a moving thing (the Moon) relative to another moving thing (the ...
Martin Kochanski's user avatar
6 votes

What is the angular diameter of Earth as seen from the Moon?

The Earth viewed from moon will appear larger, in proportion to how much larger the Earth's diameter is versus the moons diameter. Earth diameter 7900mi Moon diameter 2100mi So the Earth-view from ...
BrettB's user avatar
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6 votes
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What does the Sun look like from the heliopause?

0.004° converts to about 14.4 arc-seconds. That's within the range typical for Mars; 5-25 arc-seconds. You'll see the sun as a point with the naked eye. Heliopause is about 14 billion miles out. ...
Wayfaring Stranger's user avatar
4 votes
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How would a flat mirror on the Moon reflect sunlight

The (weird for historical reasons) defintion of magnitude is that a difference of 5 magnitudes corresponds to a factor of 100 in the brightness of the source. So a difference of 3-(-27) = 30 ...
James K's user avatar
  • 123k
4 votes

Why do we use the value 206265 in the small angle formula?

The number 206265 arcseconds/radian is often used in astronomy for angular conversions. It is simply derived from the product of 3600 arcseconds/degree and 57.2958 degrees/radian. Edit based on ...
amateurAstro's user avatar
  • 1,595
4 votes

About angular diameter, parallax and image of the nearest neutron star RX J185635-3754

The angular diameter doesn't come from imaging, but from photometry and spectroscopy. The observers estimated the surface brightness from the spectrum, and then computed how big the angular diameter ...
John Doty's user avatar
  • 1,909
3 votes

Understanding conversion to angular diameter

Given someone's address, can you find that person's height? The RA and dec is the position in the sky of an object, relative to the other stars. The angular diameter is the apparent size of the object ...
James K's user avatar
  • 123k
3 votes
Accepted

Understanding conversion to angular diameter

I was trying to better understand the concept of angular diameter and was hoping for some clarification. Given some object's coordinates in RA and dec, is it possible to find that object's angular ...
uhoh's user avatar
  • 30.7k
3 votes
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Angular diameter result confirmation

To find the angular diameter of a satellite you need to find $$\arctan\left(\frac{\text{diameter of satellite}}{\text{distance to satellite}}\right)$$ As you have been using the diameter of the planet,...
James K's user avatar
  • 123k
3 votes

What is the furthest star for which we can measure the diameter?

The parallax is always easier to measure than the angular size of any planet. This is also true for most stars, excluding hypergiants and some supergiants. The parallax is given by $ \displaystyle \...
JustThinking's user avatar
3 votes

From what distance would our sun have an angular diameter of 7 arc seconds?

While this might seem to be strictly a math problem, it's really loaded with Astronomy, let's see what we can learn! If you're far enough away that you can see essentially a full hemisphere (which you ...
uhoh's user avatar
  • 30.7k
3 votes

From what distance would our sun have an angular diameter of 7 arc seconds?

From us, the Sun is visible around a half grad, which is $\approx$ 30 arc min = 1800 arc seconds. To get to 7 arc seconds, you need to go 1800/7 times farther away, so the result is around 250 AU.
peterh's user avatar
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3 votes
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Does the given radius of a planet/moon include its atmosphere?

I looked for a source without luck, so I'll just post cause I've read this answer before. The diameter is the solid surface, not the atmosphere for rocky planets. For gas giants it's the ...
userLTK's user avatar
  • 24.1k
3 votes

Approximately what percent of the sky has nothing in it?

This is supposed to be a very rough ballpark estimate on what percentage of the sky is covered by stars in the milky way galaxy. There are approximately $10^{11}$ stars in the Milky way. First ...
quarague's user avatar
  • 191

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