63
votes
How were sundials and moondials possible 800 years ago?
As @JohnHoltz points out in a comment, planting a stick in the ground or in a wall and watching where the shadow falls is something very easy; sundials have been known since prehistoric times.
I’m not ...
- 6,701
60
votes
Is there enough matter orbiting the sun to make a second sun?
The vast majority of the stuff in the solar system other than the Sun itself is contained in one body, Jupiter. The total mass of the solar system is estimated to be about 1.0014 solar masses, or ...
- 33.4k
51
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 ...
- 2,319
48
votes
Accepted
Is the Solar core hard?
The solar core can be considered soft in a relative sense (compared to other materials at the same density), but hard and incompressible in an absolute sense. The material behaves almost exactly like ...
- 146k
45
votes
How were sundials and moondials possible 800 years ago?
Those 2 wheels work like a perfect sundial.
No they don't. They're flat. The timespan from one hour to the next on a flat sundial varies from season to season. At the same time that that temple was ...
- 33.4k
38
votes
Accepted
Is the power output at the core of the sun about the same as a compost pile (about 300 watts)?
Yes, the power output of the solar core is about 276.5 watts per cubic metre. However, if we average that power over the whole volume of the Sun it drops to 0.27 watts per cubic metre. (Thanks, ...
- 13.1k
31
votes
How to describe the Sun's location to an alien from our Galaxy?
This has already been done. The pioneer 10 and 11 probes have a description of the solar system's location and Earth engraved for aliens to understand (or so one hopes). The physical parameters of our ...
- 17k
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 ...
- 146k
28
votes
Is there enough matter orbiting the sun to make a second sun?
No. The total mass of the planets, asteroids, dust etc in the solar system is only about 0.1% of the mass of the sun. There is not nearly enough to make even a small star.
- 115k
28
votes
Why are there not a whole number of solar days in a solar year?
if the sun appears to be in the exact point in the sky as that time the previous solar year,
This statement means the sun is at (nearly) the same position relative to the stars that it was the ...
- 1,955
26
votes
Error in "Efficient Method for Calculating the time of Sunrise and Sunset" by Robin G Stuart
This is an error in the paper. The equation is based on
$\nu - M = 2\varepsilon \sin M$
Where M is simplified to be $M = l + 1.3450$ and $\varepsilon = 0.0167$, the eccentricity of the Earth's orbit .
...
- 491
18
votes
Why are there not a whole number of solar days in a solar year?
Let‘s turn your question around: Why should they be in sync?
The only mechanism that redistributes angular momentum between two celestial bodies are tidal forces (not considering general relativity ...
- 674
16
votes
Why are there not a whole number of solar days in a solar year?
It isn't in the same place.
We can measure "one year" starting at any point in time, suppose we start at sunrise on January 1st at some convenient location (eg London). We know where the sun ...
- 115k
16
votes
Is the Sun hotter today, in terms of absolute temperature (i.e., NOT total luminosity), than it was in the distant past?
From the Wikipedia article on the sun, it is about 150 K:
The Sun is gradually becoming hotter in its core, hotter at the surface, larger in radius, and more luminous during its time on the main ...
- 17k
15
votes
Is the Solar core hard?
If one thinks about "hard" as solid, having stiffness/rigidity, an ability to retain a certain shape when subjected to anisotropic stress, then the answer is NO, it is not hard.
The solid ...
- 2,776
13
votes
Accepted
What is the apparent size of Earth from the Sun?
The calculation of the apparent size of Earth as seen from the Sun is pretty straight forward. Just consider the triangle: from your observation point the distance to the center of the object, to the ...
- 17k
13
votes
Accepted
How will the expansion of the Sun influence Earth as a celestial body after 5 billion years?
Yes, the Sun will expand and will probably reach a radius of about 1 au (the current distance between the Earth and the Sun). This wil occur in just over 7.5 billion years time (not 5 billion, e.g. ...
- 146k
13
votes
Did the Sun's light always peak in the green wavelengths?
Nice question! Sun's spectral peak wavelength is currently 483 nm which falls under the category of green.
Sun's wavelength changing
In it's early days, the Sun was a lot cooler than it is today. So ...
- 1,208
13
votes
Accepted
How to describe the Sun's location to an alien from our Galaxy?
If the travel was instantaneous (or say, less than a million years) it should be relatively easy to relocate the Sun from triangulation using well-known objects visible from anywhere in the Galaxy. If ...
- 146k
12
votes
Did the Sun's light always peak in the green wavelengths?
No, but that's not why plants reflect green light
The Sun, as well as the light of nearly all stars in the universe, had their peak wavelengths shift at some point during their life.
In the case of ...
- 2,585
11
votes
How were sundials and moondials possible 800 years ago?
They were possible millenia ago as all that was needed to set them out was observation and marking of hour angles.
Equatorial sundials, i.e. a sundial whose plane is oriented parallel to the equator's ...
- 211
11
votes
Why are there not a whole number of solar days in a solar year?
Take a bike and a race track. Mark one spot on the side of one of your tires.
Now place the bike on the starting line, making sure the mark is precisely at the lowest point of the wheel (where it ...
- 461
10
votes
Accepted
What kind of nebula was the Sun formed from?
Stars and the planetary systems around them form from dark and dense molecular clouds.
Physically we distinguish
Hot ionized gas (supernova remnants and emission nebulae)
Hot neutral gas (also some ...
- 17k
10
votes
Is the Solar core hard?
It is not solid. But it is hard. Its Young's modulus is about $10^{16.5}\text{ Pa}$. It is thousands or millions of times more than the Young's modulus of any ordinary solid matter. For comparison, ...
- 183
10
votes
Accepted
How close is a typical New Moon to the Sun in the sky?
Mean angular separation of about 3.2°, based on skyfield Python library simulation of the 2nd millennium.
This is pretty dependent on your viewing location. The ...
- 4,696
9
votes
What kind of nebula was the Sun formed from?
Stars are formed in molecular clouds: these are gas clouds with conditions for $H_2$ to form.
Molecular clouds are typically dark nebulae, but if there is a bright star near them, then they can ...
- 115k
9
votes
What caused a huge piece of the Sun to break off?
Not a CME, but pretty neat. This is a prominence, the plasma in the prominence got entrailed into a "vortex" that presumably exists at the solar pole. The wind speeds in the plasma are ...
- 115k
9
votes
Accepted
Is there a (proposed) name for Coatlicue's progenitor?
It's most likely that Coatlicue didn't have an equivalent predecessor. Please bear in mind that we aren't certain that the Sun had such a single progenitor star, but it's a very good hypothesis.
Star ...
- 13.1k
9
votes
Accepted
How do people of the opposite side see the Moon and the Sun?
At the time depicted, it is close to a astronomical new moon, and night-time in Tehran. So at that time you can't see the sun nor the moon in Tehran
For people on the other side of the world (i.e. ...
- 115k
9
votes
Accepted
How did the temperature of the solar system evolve?
There is no "nuclear flash". The approach to a stable, hydrogen-burning star is rather smooth and in fact the Sun was more luminous during its first few million years as a protostar than it ...
- 146k
Only top scored, non community-wiki answers of a minimum length are eligible