planetmaker
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What ground-based observational evidence is there that the Moon doesn't have an atmosphere?
43 votes

The Moon is so close that establishment of an atmosphere would be easy: you take a photo. An atmosphere always is thicker at the surface and gets exponentially thinner outward. This would be visible ...

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Does a meteor's direction change between country or latitude?
40 votes

If you see the same meteor, it of course only has one path - even while the apparent movement differs for different observers. What you basically do in that case is triangulation - which you could use ...

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Can Pluto be seen with the naked eye from Neptune when Pluto and Neptune are closest?
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30 votes

No, it cannot. Far from it. The closest approach between both planets is roughly 16 AU due to the 3:2 orbit resonance. Pluto will even then be a tiny dot among many with a brightness around 14 mag. ...

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What rotational speed would Mercury need to have to achieve a temperature comfortable for humans
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22 votes

There is no rotation speed which can achieve that - globally. For a local analysis see below. The global equilibrium surface temperature in the absent of any atmospheric greenhouse effect and with an ...

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During solar eclipses on Jupiter, can the moon(s)' shadow(s) on the surface be seen from Earth with a telescope?
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22 votes

Yes, you can see it (I have seen Io's shadow on Jupiter and we were happy it was visitor night so that we could share the view with guests) A 50cm mirror and 125x magnification allows you to see it ...

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Can one see a black hole?
22 votes

I think, that's a bit semantics, and a question of what do you call 'I see something': If you see the shadow cast by an object on a bright background - do you see the object? Do you see Mercury or ...

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Can a satellite stay in one place but not above equator?
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21 votes

No. A geostationary satellite is in an orbit around the earth with a 24 hour period - in the same sense of rotation as the Earth rotates. That makes sure that its orientation with respect to the ...

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How to be an astrophysicist?
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20 votes

In addition to the answer of James K, who outlines the most straight forward way into astrophysics, there's many paths. Some others include: There are people who did a BSc and/or MSc in Engineering ...

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Can we change Earth's orbit with modern technology?
19 votes

No, we cannot. It's relatively simple math to show that we are a very long shot from changing Earth's orbit by anything significant at all: consider the kinetic energy of Earth in its current orbit, ...

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Was this an astronomical phenomenon observed in 1689?
18 votes

That is hard tell; from your description it reads like a spectacular halo phenomenon. They are more common when the sun is still up in the sky, but even after sunset many are possible and they can ...

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Can we visually perceive exoplanet transits with amateur telescopes?
16 votes

Amateur equipment is good enough. But you cannot detect it with a naked eye. The change in flux for a passing exoplanet in transit is roughly 1%...2% at most for the larger exoplanets - and it is a ...

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Why aren't there any images of Sun-orbiting bodies by Spitzer?
14 votes

All "big" instruments have observation logs, so does Spitzer. The complete logs are here but there's also a filtered log for solar system observations which shows basically all planets and ...

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How is an exoplanet characterised as "Earth-Like"?
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13 votes

There is no unique definition of Earth-like and it depends on what characteristics are important in the context of discussion. The most simple one is to just compare radii and masses. That is ...

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What is the mnemonic reason behind b being galactic latitude? (in the Galactic Coordinates frame)
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12 votes

Länge und Breite are the German words for longitude and latitude. Thus 'b' seems like a natural choice for latitude and 'l' for longitude. A century ago when traditional choice of variable names were ...

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If we could fly off edge of observable universe what would happen?
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11 votes

The 'edge' of the observable universe is as much a edge as is the 'edge' of how far you can look from the roof of your house: none at all, it's just a limit to our vision. We can never reach this edge ...

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Semi major axis of an hyperbolic orbit
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11 votes

A semi-major axis of an ellipse defines the half diameter of the ellipse where it is longest. That's a convenient measure to compare ellipses and express its properties. Mathematically you can ...

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Why can't we see Saturn's phases from earth?
11 votes

The answer is simple geometry: we are 1 AU from the Sun, Saturn is 10x further from the Sun. Looked at Earth from Saturn, Earth is always in front, behind or immediately next to the sun; an observer ...

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Why is Mercury's Density So Low?
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10 votes

The actual density depends on the mineralogy, we don't have a crystalline iron core and silicon crust. You do have a lot of oxygen available, too when you look at the overall elementary abundance. So ...

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How do comets' nuclei stick together?
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10 votes

Van-der-Waals forces and gravitation are the main answer. The main property keeping together bodies smaller than a few $100 {\rm km}$ are their constituents surface forces between the grains (van-der-...

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How do I say the luminosity class aloud?
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9 votes

The luminosity classes are indicated by Roman numbers. So you pronounce them as numbers if you don't spell out the actual name of the luminosity class you are referring to. Pronouncing them as letters ...

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How can a black hole have a charge, or be charged?
9 votes

The answer is reasonably simple: (nearly) all matter consists of protons, neutrons and electrons, thus of either positively, or negatively charged particles or those with no charge. The next step you ...

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What is the definition of a "pole" of a celestial body?
9 votes

You are mixing the rotation of the body around a barycenter with its moons, and the rotation of the body around its own center. For a bound rotation like Pluto and Charon both have to have the same ...

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What does "unremarkable transverse peculiar velocity" mean exactly, and how is it calculated here?
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9 votes

"Peculiar velocity" is a fixed term and describes the velocity of an object relative to a defined rest frame. Astronomy has the problem that you need different methods to measure the 3D ...

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What would Jupiter look like from a Galilean moon?
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9 votes

You can simply startup Stellarium and have a look yourself. Choose any locations you are interested in. The Galilean satellites of Jupiter are inside the default list of locations. The attached image ...

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How big is the galactic magnetic field in the solar system and how can we measure it?
8 votes

There is no galactic magnetic field inside the (inner) solar system. The solar wind is a near-perfect plasma and according to the frozen-in theorem it carries the magnetic field of the Sun with it ...

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Why can space telescopes see through a planetary nebula?
8 votes

The answer is quite simple: you can also see the other side of the room (or maybe through the fume or haze above your oven) when you create a tasty meal) - even when the space between you and the wall ...

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Is looking through a telescope to do astronomy a thing of the past?
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8 votes

Scientifically there's little to gain when you look through a telescope with your own eyes. Attach a camera to the telescope and you immediately document what you observe and take out the subjective ...

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Are Brown and Sub-Brown Dwarfs secretly more common than stars?
7 votes

This is an important question to ask about the initial mass function of objects in the Galaxy - and the final answer hasn't been cast as it is a matter of research. Yet, observational data (e.g. see ...

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How do astronomers know that distant galaxies aren't actually nearby star clusters?
7 votes

We measure the distance. A cluster is a group of stars in close proximity to each other. For relatively near stars you can directly measure the distance via the parallax. That's what the Gaia ...

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Hydrogen burning vs Hydrogen fusing
7 votes

Yes, it's the same thing. Usually the phases a star goes through (if big enough) are the hydrogen burning helium burning ... and the phase is named after the element which is being fused into a ...

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