35
votes
Accepted
What's the difference between minutes and arcminutes?
This can get a bit confusing, because "arcminute" and "minute" are both sometimes used in celestial coordinate systems but mean two different things.
An arcminute is 1/60th of a ...
- 36k
16
votes
What's the difference between minutes and arcminutes?
Your trigonometry book isn't wrong: both "minute" and "arcminute" can refer to $\frac1{60}$ of a degree. It's certainly a very good idea to use the term "arcminute" when ...
- 13.1k
7
votes
Accepted
Measuring misalignment between two positions on sky
Assuming you mean the angle between the meridian line through A and the great circle that goes through points A and B, then it goes something like this.
Define vectors from the origin to A and B ...
- 146k
7
votes
Accepted
What is the area of the Summer Triangle?
The area of a triangle enclosed by 3 stars on the celestial sphere, in square degrees, is given by:
$$ A = \frac{180}{\pi}\times E$$
Where E is the spherical excess and is equal to the sum of all ...
- 888
6
votes
Right Ascension for epoch 2000 - physical location?
Fortunately things aren't as entangled as you thought.
The J2000 epoch does not depend on the equinox; it is simply 12:00 TT on January 1, 2000.
The time of vernal equinox is unrelated to sunrise ...
- 18.2k
6
votes
Accepted
Right ascension and sidereal time difference
Question 1: How do sidereal time and RA correlate together?
Think of the sky as a globe. The constellations and lines of right ascension (RA) are painted on the globe. Because of the Earth's rotation,...
- 7,787
6
votes
Accepted
Right Ascension of North Celestial Pole
The north celestial pole doesn't have a right ascension. It's like the north terrestrial pole not having a longitude. Asking for it creates an exception. In any case, the RA of a point, other than the ...
- 1,608
6
votes
Accepted
Is the Ra/Dec of Alnitak in Orion's Belt known to be correct?
I tried to reproduce your result. I took the values of RA and DEC from Wikipedia: Alnitak, Alnilam, Mintaka.
The values are the following:
Alnitak: Ra = 05h 40m 45.52666s, Dec = −01° 56′ 34.2649″
...
- 4,371
5
votes
Calculating the time a star is at my local meridian based on its right ascension
The sun's position is irrelevant to when a star will be in its meridian; the Sun's coordinates in the RA/dec grid does change over the year. A star's position is fixed in that system and it will be ...
- 17.1k
5
votes
Accepted
Is this how stars’ right ascensions correlate to planets’ longitudes on a 2d map?
Most of your assumptions are correct.
However, right ascension and declination are an equatorial system, while (heliocentric or geocentric) longitude and latitude are an ecliptic system. The reference ...
- 6,701
5
votes
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Does RA/Dec vary with the Earth's orbit around the Sun?
This is called "parallax". The observed position of a star (as measured by RA and Dec) does vary ever so slightly.
For example, Proxima centauri, the closest star has a parallax of 0.77 ...
- 115k
4
votes
Accepted
Convert a Decimal into RA or Dec
DecRA is the decimal right ascension
RAh, RAm, RAs are the hms form
$${\rm DecRA} = {\rm RAh}\times 15.0 + {\rm RAm}/4.0 + {\rm RAs}/240.0$$
$$ {\rm RAh} = {\rm INT}({\rm DecRA}/15.0) $$
$${\rm RAm} =...
- 146k
4
votes
Measuring misalignment between two positions on sky
The position angle P of a body ($\alpha_1, \delta_1$) with respect to another body ($\alpha_2, \delta_2$) can be calculated from
$$tan(P)={sin(\Delta\alpha)\over cos(\delta_2)tan(\delta_1)-sin(\...
- 7,787
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
Accepted
Is the vernal equinox always in zenith somewhere on the equator?
The Vernal Equinox is defined by the point where the sun's path across the sky, the Ecliptic, crosses the Celestial Equator. It is always going to be on the Celestial Equator.
Given the various ways ...
- 4,696
4
votes
Accepted
What is meant by *topocentric* right ascension and declination?
You are correct that topocentric coordinates are for the position of "close" objects, corrected for observing from the Earth's surface instead of the theoretical center.
The topocentric and ...
- 7,787
4
votes
Accepted
At what time of the day of autumnal equinox the Sun's equatorial coordinates reach max value?
As noted in comments, at the september equinox, the sun's declination is decreasing (slightly) during the day, so the declination will be slightly higher in the morning than in the evening.
But the ...
- 115k
4
votes
What's the Right Ascension & Declination of Galactic Left & Galactic Right?
I think it makes sense to first think of the directions not in terms of equatorial coordinates (right ascension and declination) but in galactic coordinates (galactic latitude and galactic longitude, ...
- 36k
4
votes
Accepted
Sidereal time of rising and setting of the sun on the arctic circle
You need to intepret the question a bit. By "June 22" they probably mean "Summer Solstice", likewise for "Dec 22". At the solstice the right ascension of the sun is 6hr/...
- 115k
3
votes
Right Ascension for epoch 2000 - physical location?
There are several meanings to "vernal equinox", and I think you have not picked the right sense here.
The vernal equinox is here taken to be a point on the celestial sphere. It is the point where the ...
- 115k
3
votes
Accepted
Help with Converting Right Ascension and Declination into 3D Vectors
The stars should fit $r = \sqrt{x^2 + y^2 + z^2}$ but seem to be plotted in the $x = r$ plane instead.
Conventionally the x axis is at (α=0h, δ=0°),
the y axis is at (α=6h, &...
- 18.2k
3
votes
Moon equatorial coordinates
It's a bit late but hopefully it will help others in the future.
The calculations can be found on this site, article from Keith Burnett ([email protected])
The first problem is that you use ...
3
votes
Accepted
When does a star rise with the sun?
The formula you gave is to find the hour angle of the star while setting, not the setting time itself. Suppose the hour angle of star is $HA\star$, and $RA=\alpha$,then the Local Sidereel Time is ...
- 266
3
votes
Accepted
Centre (projected) of a galaxy cluster
Short answer, no, they don't.
Longer answer, it's complicated. There are, in essence, five different measures for the centre of a galaxy cluster, based on different physical properties of the cluster,...
- 2,275
3
votes
Accepted
How can I calculate the right ascension of an observer on Earth's surface in the TEME frame with python.skyfield?
An Earth-centered observer looking at that point on Earth’s surface would also be looking toward the point that is the zenith for that location on Earth.
Thus, you just need to find the local sidereal ...
- 4,864
3
votes
Accepted
Calculating the time a star is at my local meridian based on its right ascension
Note: I'm accepting this as the answer because the main cause of the problem was that I was counting the days wrong. But there is another more important, more fundamental problem which was correctly ...
- 211
3
votes
What is the rate of change by day for a given right ascension?
Answer to your question
RA changes just a little bit through time (because of minor effects like parallax), but we can just say it is constant for a specific non-Sun star. Same applies to declination. ...
- 2,879
2
votes
Accepted
Declination calculation
You are forgetting parallax.
Something which is very distant and lies on the celestial equator will have a declination of 0°, but if it is nearby then its declination will only be 0° to an observer ...
- 1,345
2
votes
Accepted
Using setting circle for Right Ascension
I think I mis-read your question before. Let me rephrase it to make sure I understand. In step D (moving the scope from Sirius to M42), that process took 5 minutes to complete. Or maybe it took 30 ...
- 7,787
2
votes
Accepted
Presenting / printing Right Ascension and Declination values
There are no standards for representing right ascension and declination apart from the abbreviations RA and Dec, and the symbols $\alpha$ and $\delta$.
The use of decimal degrees, dms, °'", hours and ...
- 1,345
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