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52 votes
Accepted

How much of the surface of other planets is lit by the sun?

OK, let start with some assumptions: spherical objects and no atmospheric effects. Here's the relevant geometry with the object on the left representing a planet and the object on the right ...
GrapefruitIsAwesome's user avatar
46 votes
Accepted

Why wasn't the planet Uranus recognized by ancient cultures?

The source of light that is Uranus was observed as far back as 128 BC. However it was misidentified as star as late as the 1760's. Then it was observed by Herschel in 1781, who also misidentified it,...
StephenG - Help Ukraine's user avatar
32 votes
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Why is it impossible to infer the surface temperature of Venus by spectroscopy observation from earth?

The important difference is that the word "surface" has a different meaning depending on whether we're talking about the Sun or Venus. When we say "the surface of the Sun," we ...
Tanner Swett's user avatar
29 votes
Accepted

Why did the dust between the planets disappear during the birth of the solar system?

Dust happens in two ways. "Primordial dust" just condenses out of the protostellar material in the disc providing it gets cool enough and dense enough. "Second generation" dust is ...
ProfRob's user avatar
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27 votes

Why wasn't the planet Uranus recognized by ancient cultures?

I like ScienceSnake's comment - a combination of faintness and its comparatively small apparent motion. It was observed in 1690 by John Flamsteed on several occasions, but because it moved so little ...
Bumptious Q Bangwhistle's user avatar
18 votes

Why is it impossible to infer the surface temperature of Venus by spectroscopy observation from earth?

I think the problem is that spectroscopy at visible and infrared wavelengths simply can't penetrate the thick atmosphere of Venus. Thus the information comes from much higher in the atmosphere and not ...
ProfRob's user avatar
  • 155k
17 votes
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Visibility of Planetary alignment of 6 planets on 3 June 2024 from Prague

Not in a meaningful way. Saturn will be visible and a bright object in the morning sky. Mars should be visible, but by the time it rises, twilight will have started. Jupiter and Mercury are only a few ...
James K's user avatar
  • 125k
17 votes
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What can I see with 10x50 binoculars in bortle class 4?

This planet "parade" is over-hyped. There is little to see observationally, since Jupiter and Mercury are less than 5 degrees above the horizon when the Sun rises, Uranus is about 10 degrees ...
amateurAstro's user avatar
  • 1,790
16 votes

Why wasn't the planet Uranus recognized by ancient cultures?

It would require a lucky coincidence As noted by others, Uranus is faint and easy to misidentify as a fixed star. However, Bourtembourg 2013 claims that Uranus was observed already in 128 BC by ...
Martin Modrák's user avatar
13 votes
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Why can't a half-illuminated planet support life at all?

The phenomenon you are referring to is called "tidal locking", it occurs when the orbital period or one revolution of the planet is roughly equal to the time it takes to rotate in on its ...
Arjun's user avatar
  • 1,624
13 votes

Visibility of Planetary alignment of 6 planets on 3 June 2024 from Prague

James K is correct, but I would like to add some details on the "where can I find out" part of the question. If you are ever interested in what objects (stars, sun, moon, planets, satellites,...
SpaceCore's user avatar
  • 3,441
12 votes

Why can't a half-illuminated planet support life at all?

Pierrehumbert (2010) modelled the possible climates of Gliese 581g. Although that planet has since been shown to probably not exist, the model of a planet with similar characteristics should still ...
gerrit's user avatar
  • 2,233
11 votes
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Is there a formula for calculating surface temperature of a rocky world based on atmosphere and solar input?

Guillot et al. 2010 give a comparatively simple solution (their Eqn. 27) for a double-gray purely radiative atmosphere. It reproduces the equilibrium temperature in the optically thin limit. Robinson &...
AtmosphericPrisonEscape's user avatar
9 votes
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Celestial "orbits"

Physical theories describe how things change in given circumstances (if the theory is right). In practice this means that they are applied in simplified ways, where the simplifications of ...
Anders Sandberg's user avatar
9 votes

How is a Planet's Moment of Inertia Measured Remotely?

I dug around quite a bit and was unable to find anyone making measurements of the type that would lead one to determine the moment of inertia of Uranus or Neptune without first making assumptions ...
Paul T.'s user avatar
  • 727
7 votes

Difference in results between JPL Horizons and cspice (rust-spice)

The issue is with how you have configured HORIZONS to show the output. You have apparent RA, Dec selected as output columns which includes the precession and nutation, in addition to the light time ...
astrosnapper's user avatar
  • 8,442
7 votes
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Does the Sun have different unique orbits around barycenters?

There's only one barycenter for the solar system, which includes the impact from every planet, asteroid, comet, and space probe in the solar system. The path the sun takes around that barycenter is ...
Darth Pseudonym's user avatar
6 votes

Why don't scientists believe Earth's surface water came up from the mantle?

If the question is actually "why don't any geologists believe that Earth's surface and near-surface waters were brought up from the Earth's interior?", then the premise is false since many ...
David Bailey's user avatar
6 votes

Celestial "orbits"

You are right. But you miss a few quirks of reality, or some fine-print of the applicable physics. Yes, the Moon is moving away from Earth as slowing down Earth's spin rotation means that angular ...
planetmaker's user avatar
  • 19.9k
5 votes

Why can't a half-illuminated planet support life at all?

As several people have indirectly noted, the premise of this question is controversial. There are plenty of studies* suggesting that an Earth-like planet tidally locked to its star can be habitable as ...
Ilmari Karonen's user avatar
5 votes
Accepted

When did Venus reach approximate radiation equilibrium?

If we assume that energy transport in atmospheres happens purely via radiation, then we can estimate a simple adjustment time to an exchange of exterior forcing factors (change in mean solar angle, ...
AtmosphericPrisonEscape's user avatar
5 votes

Why can't a half-illuminated planet support life at all?

I wish to add a couple of comments to the good answer provided by Arjun. A popular fallacy has it that close-in planets are always tidally locked in a 1:1 spin-orbit resonance (SOR), i.e., are always ...
Michael_1812's user avatar
  • 1,494
5 votes

What color is the Sky on other Planets?

On Mars, it's pink because of the rust in the air. But it's blue near the sunset, where light doesn't get to bounce off the iron and be reflected red. On Venus, it's yellow because of sulfuric acid ...
Miss_Understands's user avatar
4 votes

How common for all 7 planets to be above the horizon?

This is definitely dependent on observing location, observing times, atmospheric refraction, and probably a couple other things I didn't think of. But just to get another reasonable estimation: ...
notovny's user avatar
  • 4,786
4 votes
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When did Venus get as hot as it is now?

According to current models and computer simulations, Venus was a water world many billion years ago but due to the close proximity to it's host star, Sun or Sol, the water was Photocatalytically ...
Arjun's user avatar
  • 1,624
4 votes

Celestial "orbits"

The model of the elliptical orbit is a close approximation of the reality—and even Kepler knew that, when he wrote that “orbits are ellipses.” In reality, gravitational interactions between the Sun, ...
Pierre Paquette's user avatar
3 votes

For what period of time can planets be viewed from the North Pole as just being there?

Take Jupiter for example. It is currently (as of Oct 2023) visible from the North pole continuously (it is circumpolar). The sun has now set at the North Pole, and right now Jupiter is above the ...
James K's user avatar
  • 125k
3 votes
Accepted

Is the magnetic permeability (mu_0) necessary in the expression for planetary magnetic field?

It entirely depends what system of units you are working in. $\mu_0 \simeq 4\pi\times 10^{-7}$ in S.I. units. However, in Gaussian units, $\mu_0=1$ and so it isn't included. In Gaussian units, the ...
ProfRob's user avatar
  • 155k
3 votes

Is Proxima Centauri a Planet?

Per the IAU definition: A "planet" is a celestial body that: (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a ...
James K's user avatar
  • 125k
3 votes

If an orbit is shifting due to orbital precession, is it still a Keplerian orbit?

To the good and thoughtful answers given above, I would add some general considerations. While a Keplerian orbit exists only within a two-body problem, in a perturbed setting an actual orbit is ...
Michael_1812's user avatar
  • 1,494

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