52
votes
Accepted
Why won't the Sun set for days at N66.2 which is below the arctic circle?
From the PyEphem Quick Reference Guide:
Rising and setting are sensitive to atmospheric refraction at the
horizon, and therefore to the observer’s temp and pressure; set the
pressure to zero to turn ...
- 4,696
35
votes
Accepted
JPL Horizons sending too many emails
It turns out the "more robust" mail queue manager implemented April 21 has a condition where it can rediscover itself. That condition occurred in the middle of the night local time with your ...
- 466
28
votes
Accepted
Why does the Earth have a z-component at the start of the J2000 epoch?
You made the same fundamental mistake that Anton Gromov made in his question on the sister Space Exploration StackExchange network site: You used the solar system barycenter rather than the Sun as the ...
- 33.4k
22
votes
Why won't the Sun set for days at N66.2 which is below the arctic circle?
Correct me if I am wrong, but if we count sunsets by the center of the Sun apparently crossing the horizon then the Sun is supposed to set every day at latitudes under the arctic circle.
That is not ...
- 33.4k
22
votes
Accepted
How do planetarium apps and software calculate positions?
There are quite a few different types of methods of computing the position of celestial objects, and the method used to compute the position generally depends on the type of object and how accurate ...
- 5,542
15
votes
Accepted
JPL Horizons - "highly accurate measurements of planetary positions" - how do they do it?
The JPL ephemerides are good, but they certainly are not precise to the fraction of a millimetre!
From the Horizons docs:
Statement of Ephemeris Limitations
To produce an ephemeris, observational ...
- 13.1k
14
votes
Why is the time between equinoxes different?
This is because the summer and winter solstices (approx. June 21st and December 21st) do not correspond to the aphelion and perihelion (approx. July 5th and January 4th). Therefore, the average ...
- 4,780
12
votes
Accepted
What unit of year in JPL Small-Body Database Lookup?
Based on the Astrodynamic Parameters page, It appears to be the astronomical Julian year, defined as exactly 365.25 days of 86400 SI seconds.
- 4,696
12
votes
How do planetarium apps and software calculate positions?
How do planetarium apps and software calculate positions?
The generic answer is that they use some form of ephemeris. The VSOP87 is an example of an ephemeris. So is JPL Horizons, which uses ...
- 33.4k
11
votes
JPL Horizons - "highly accurate measurements of planetary positions" - how do they do it?
@PM2Ring's answer does a great job of explaining that folks who do careful numerical integration keep a large number of digits beyond the final accuracy because if they didn't, after tens of thousands ...
- 31.4k
9
votes
Why won't the Sun set for days at N66.2 which is below the arctic circle?
Wikipedia's article on the Arctic Circle provides the explanation. Firstly, it says:
because the sun appears as a disk and not a point, part of the
midnight sun may be seen on the night of the ...
9
votes
Accepted
Earth-Moon Barycenter Perihelion
From a comment by the OP,
How do I set it to the center of the Sun?
Select Vector table as the ephemeris type. Choose the Earth-Moon barycenter as the target body and @sun as the coordinate center. ...
- 33.4k
7
votes
Instantaneous gravity in equation of motion for ephemerides calculation
Would this not mean that information travels infinitely fast and thus contradict Relativity?
Look more closely at equation 27 in the referenced document. I'll simplify this as
$$\boldsymbol{\mathrm a}...
- 33.4k
7
votes
Accepted
Question about Cowell's method
Yes, that interpretation is correct. The given formula only denotes the (integral) acceleration seen by a single mass element at a certain time $t$ (as implicitly $\mathbf{r_i} = \mathbf{r_i(t)}$ ...
- 17.1k
7
votes
How to get JPL Ephemeris data to work in n-body simulation or why is Mercury flying to Jupiter
I used this n-body algorithm.
Do not use that algorithm, repeated many times over. As a starter, the algorithm uses symplectic Euler. Paraphrasing from the movie "Jaws", "You're going ...
- 33.4k
7
votes
Accepted
Different results for the same JPL Horizons query
The issue is with differences in how the defaults behave between the code and the website. In the code version of the API id=3 gives the Earth-Moon Barycenter (as ...
- 8,313
7
votes
How to test accuracy of DE441?
I'll start at the end and walk your question backwards to provide a partial answer. However I too am looking forward to find out if there is any independent testing of the DEs using phenomenon not ...
- 31.4k
6
votes
Apsides Calculation
For those who don't have ready access to a copy of Astronomical Algorithms,
Meeus's first approximation looks like:
$$ \text{JDE} = 2541547.51 + 365.259636 ~k + 1.6 \times 10^{-8} ~k^2 $$
where k, the ...
- 18.2k
5
votes
Accepted
L2 point ephemeris (celestial mechanics)
Bad news, this type of SPK file has a different sort of interpolation that is not supported by the jplephem package (Hermite interpolation vs Chebyshev polynomials)...
- 8,313
5
votes
Accepted
Why is the time between equinoxes different?
The diagram below illustrates the reason, which is that the earth moves at different speeds during it revolution around the sun and the distribution of those speeds is not equal because the solstices/...
- 883
5
votes
Accepted
Predicting accurate time of local sunrise in mountainous area?
The PeakFinder app1 can do most of what you're looking for. It's available for Android and IOS as well as through a web browser. The app shows the paths across the sky for both the sun and moon2 and ...
- 1,189
5
votes
Accepted
Implementing the VSOP-2000 Ephemeris
Probably the most reliable guide to vsop2000-p11.dat is the reference implementation vsop2000.for.
The input line in question is ...
- 18.2k
5
votes
Instantaneous gravity in equation of motion for ephemerides calculation
Short answer is "no".
I can model gravity by Newton's law of gravitation, and it gives extremely accurate results in most situations. Even though it is an approximation to the more accurate ...
- 115k
5
votes
Is there any ephemeris generator that gives you accurate distances between Earth and other bodies?
JPL HORIZONS gives the distance from the observer to the target as delta.
In Table Settings this option is 20. Observer range & range-rate.
For asteroids and ...
- 18.2k
5
votes
Apsides Calculation
Do you need 1000 years? 50 years? 1-hour accuracy? 1-second accuracy?
A simple linear interpolation for the perihelia from the year 2000 to 2050 gives a maximum error of about 1.3 days for the year ...
- 256
5
votes
Accepted
Linear ephemeris
This is a fairly standard ephemeris for e.g. times of maximum light of a variable star but you are getting some of the symbols mixed up. The ephemeris is normally written as:
$$
T_{event} = T_0 + P\...
- 8,313
4
votes
How can we map masses orbiting the outer Solar System by determining the orbits of planets and smaller objects?
Iorio (Preliminary constraints on the location of Telisto/Planet Nine from planetary orbital dynamics) has suggested transmissions from the New Horizons probe (currently beyond Pluto) could be used to ...
- 2,467
4
votes
Accepted
How to calculate the ground track of the Moon's position on the Earth's surface?
Given a date and time, the position of the Moon can be calculated to provide the declination and right ascension. The sub-point of the Moon (the point on the Earth at which the Moon is at the zenith) ...
- 7,787
4
votes
How can I calculate 27 tropical/solar year since my birthdate?
From the Wikipedia page on the Tropical year,
The mean tropical year is approximately 365 days, 5 hours, 48 minutes, 45 seconds.
Starting from the Tropical year proceeding after your birth date, ...
- 262
4
votes
Understanding the JPL Ephemerides
There is a python package called Skyfield that loads, reads and interpolates the binary forms of the JPL Development Ephemerides or DEs for you, and does everything else you need to get the absolute ...
- 31.4k
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