107
votes
Accepted
If Earth is tilted, why is Polaris always above the same spot?
You are correct that the axis of the Earth's rotation is tilted with respect to the plane of its orbit by 23 degrees. But it is incorrect that the direction that the axis points changes by a large ...
- 8,062
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 ...
- 16k
64
votes
Accepted
How old is the oldest light visible from Earth?
The oldest light in the universe is the cosmic microwave background. Roughly 380,000 years after the Big Bang, protons and electrons "recombined"1 into hydrogen atoms. Before this, any photons ...
- 37.3k
53
votes
Accepted
Why are the time zones calculated as 360°/24 and not 361°/24 or 360°/23.933?
The Earth takes 23 hours 56 minutes to rotate once. But that is not relevant to most people. Sure, the stars will be in the same position again after 23 hours 56 minutes, but the sun will not be in ...
- 127k
51
votes
Why is the solstice the first day of summer, and not the central day?
The English word "summer" means the season of the year that is associated with higher temperatures and shorter nights. There is no official "first day of summer" and different ...
- 127k
47
votes
Accepted
Why is the summer solstice night shorter than the winter solstice day?
Why is the summer solstice night shorter than the winter solstice day?
Aside: If you use your sunrise / sunset calculator you will see that day and night are not equal at the equinoxes. The reason ...
- 34.4k
45
votes
Accepted
Why does the Moon appear gray when passing between the Sun and the Earth?
That's what it really would look like if you were there with DSCOVR. The albedo of the Moon is only about 0.136, about half of the Earth's average albedo. Of course the part with clouds is higher.
I ...
- 30.6k
45
votes
Accepted
Why is there a year 1 B.C., a year 0, and a year 1 A.D. in NASA’s Five Millennium Canon of Solar Eclipses?
This is all explained on the home page All dates are astronomical dates. These match CE (or AD) dates for positive values but differ by 1 for BCE dates.
The Gregorian calendar is used for all dates ...
- 127k
42
votes
Accepted
What does the Earth look like when viewed from Saturn?
Yes!
It's a tiny blue dot of just about 1.6 arc seconds diameter. Cassini made the most famous image of the "In Saturn's Shadow – The Pale Blue Dot", in remembrance of the first such image ...
- 20.4k
41
votes
Simple experimental evidence that Earth revolves around Sun
The answer is ironic: Without good instruments, there is no evidence. The people who thought that the Sun went around the Earth were perfectly correct as far as the actual evidence went until the ...
- 7,670
38
votes
Accepted
Can an object from a natural process escape earth gravitation?
Yes, it is not only possible, but has almost certainly happened on Earth. The asteroid that killed the dinosaurs is thought to have produced these high velocity fragments on the order of one-...
- 16.3k
37
votes
Why are there no stars visible in cislunar space?
It is a matter of exposure and dynamic range. A sensor like a camera can only handle inputs in a certain range of intensities, and much of photographic skill (or smart presets) is about mapping the ...
- 15.2k
37
votes
Accepted
Was the Geocentric Model correct at all?
Ptolemy's epicyclic, geocentric model, in use until the Renaissance, was very accurate in terms of predicting the positions of planets and the times of eclipses. What it couldn't account for were ...
- 158k
32
votes
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Official degrees of earth’s rotation per day
First, we need to decide which definition of "day" to employ. There are several types of days:
Apparent solar day: the time between two successive culminations of the Sun (apparent Noon) from an ...
- 1,016
32
votes
Why are perihelion and the shortest day in the northern hemisphere different?
We have "days" because the Earth is rotating. The shape of the orbit has little effect on the daily cycle of light and dark.
The varying length of the day is determined by the tilt of Earth ...
- 127k
31
votes
Is it at all possible for the sun to revolve around as many barycenters as we have planets in our solar system?
The short answer is no; there is only one barycenter. Yes, you can count the Sun/Jupiter barycenter or the Sun/Saturn barycenter, or whichever barycenter you want, but the net effect of all Solar ...
- 7,614
30
votes
Why are the time zones calculated as 360°/24 and not 361°/24 or 360°/23.933?
We teach the students:
Sidereal day: In 23 h 56 min the earth rotates 360°
Solar day: In 24 h the earth rotates 361°
You should not teach your students that.
You should instead teach your students ...
- 34.4k
30
votes
Why is the L3 Lagrangian point not perfectly stable? And why is the Earth-Sun L3 point a bit less than one A.U.?
L1, L2 and L3 are saddle points in the effective potential of the gravitational field in a rotating frame of reference.
That is if you combine gravity (of Earth and Sun) with the centrifugal force on ...
- 127k
29
votes
Accepted
Safest Place on Earth from Falling Meteorites
Robertson et al. (2021) (note the erratum) provide a very comprehensive answer to your question.
Surprisingly (to me) the asteroidal flux (impacts per unit area) is slightly higher at the poles (by 8%)...
- 158k
28
votes
If suddenly "knocked" or perturbed from its orbit, would gravity eventually return the Earth to its original orbit?
There's a few parts to this question so there's more than one answer.
Earth gets knocked a little bit out of its orbit all the time by gravitational influence of other planets in our solar system. ...
- 24.2k
27
votes
Why would it be very light out at 3 AM?
I come from a similar latitude and enjoyed the night skies and two things that can add to "the amount of light in the sky" that I can think of. The first is the near-full moon as mentioned ...
- 30.6k
26
votes
Accepted
Would the Earth and Moon still have tides after the Earth tidally locks to the Moon?
There won't be significant lunar tides. The moon will become fixed at one location above the Earth. The tidal forces that drive the resonant water flows that we call "tides" will be fixed. ...
- 127k
25
votes
Which is brighter, Mars as seen from Earth, or Earth as seen from Mars?
We can use the expression that is commonly used to estimate the apparent magnitude of a planet or asteroid in the Solar System:
$$\boxed{m=5 \log \frac{1329}{d \cdot \sqrt p}+5 \log (D_s \cdot D)-2.5\...
- 2,264
25
votes
How one can establish that the Earth is round?
Observer the shadow of the Earth on the moon. It is circular, no matter where the moon is in the sky.
Observe that the position of the stars in the sky changes as you travel north or south. In ...
- 127k
25
votes
Accepted
How high does the ocean tide rise every 90 minutes due to the gravitational pull of the space station?
Bigger than an atomic nucleus but smaller than an atom.
The tidal forces scale as $MR^{-3}$, where $M$ is the mass of the object causing the tides and $R$ would be its separation from the centre of ...
- 158k
24
votes
Accepted
If Theia really crashed into Earth, did it impact Earth's axial tilt?
TL;DR: Yes
Long answer:
Theia is a hypothesized ancient planet in the early Solar System that, according to the giant-impact hypothesis, collided with the early Earth around 4.5 billion years ago, ...
- 5,191
24
votes
What is the difference between New Moon day and Lunar Eclipse?
A new moon is also when the Earth is in between Sun and Moon.
You got this one wrong. The description above applies to full moon. Correct is:
A new moon is when the Moon is (roughly) in between Sun ...
- 1,679
24
votes
Accepted
Earth is Tilted, but in reference to What Object / relative to what?
The orbit of the Earth around the Sun defines an orbital plane.
The equator of the Earth is (currently) tilted at an angle of 23.4 degrees to this orbital plane.
Or to put it another way, the spin-...
- 158k
23
votes
Did the earth form outside the solar system?
The chance happenstance that Earth happened to be floating along and got captured is minuscule. How did Earth wind up floating through space? There's no established mechanism for terrestrial planets ...
- 15.1k
Only top scored, non community-wiki answers of a minimum length are eligible