|WikiProject Astronomy||(Rated Start-class, High-importance)|
- 1 Is this way of calculating it correct?
- 2 To understand the origin of the expression right ascension
- 3 What A Terrible Diagram
- 4 Rewrites in the second para .
- 5 Give the concept meaning
- 6 Is really how you measure the zero point of Right Ascension?
- 7 Great article
- 8 Early notation
- 9 Error in example?
- 10 Sidereal hour angle
- 11 Aries vs Pisces?
- 12 Comparability
- 13 h:m:s in radians?
Is this way of calculating it correct?
If you are on the Tropic of Cancer and looking directly straight up, at the zenith, on the March equinox, then the object is at 0 hours R.A.
If you wait an hour and look up it is at 1 hour R.A.
Is this correct?
Similarly, if you wait a month, 1 year/12 days, and do the same thing then it is at 2 hours.
- No, it's not right. At the Vernal Equinox, the right ascension (RA) is 0o; at the Summer Solstice, the RA is 90o; at the Autumnal Equinox, the RA is 180o; at the Winter Solstice, the RA is 270o; and at yet another Vernal Equinox, the RA is 360o, or back to 0o. In other words, it completes a 0o - 360o cycle in one year, NOT in one day. That's to say, it takes the RA roughly 3 months to advance 6 hours, or 1 year to advance 24 hours. And keep in mind that the angular speed of RA is not uniform in time.
- One way to avoid confusion in visualizing either the Equatorial Coordinate System or the Ecliptic Coordinate System is to assume that the Earth does NOT spin, or rotate, around its own axis but only translates along its orbit around the Sun.
- --Roland 02:40, 19 February 2012 (UTC)
To understand the origin of the expression right ascension
The altitude-azimuth (alt-az) coordinate system is appropriate for localization of terrestrial objects but the earth's rotation causes celestial objects to move with time both in altitude and azimuth at a variable rate making it really difficult to "track" objects. Alt-az coordinates of an object vary with time and are different for different places on earth.
It is very convenient to consider a system in which only a single coordinate depends on time. One way is to tilt the alt-az mounting of the observing instrument by an angle equal in order that it is aligned parallel to the earth's axis. This polar aligned axis is called the declination or dec axis which now points toward the celestial pole instead of the zenith. Now once a sky object is located it can be tracked with a single motion (the old azimuth motion).
Declination runs from d=+90° at the north celestial pole through zero at the celestial equator to -90° at the south celestial pole. A given declination is represented by a circle of declination on the celestial sphere except for zero declination, the greatest circle called the celestial equator. At latitude f declination d=f is always on the zenith.
The meridian circles of earth localization system are replaced by great circles of hour angles (HA) measured east (or west) of the local meridian. Each hour corresponds to 15° of arc along the celestial equator. The projection of the local meridian on the celestial sphere is zero hours. Rising objects are east and setting objects west. Due east on the eastern horizon (which contains the celestial equator) is six hours east, the western horizon six hours west. A celestial object with HA 2 hours east (HA=2h00mE) will cross the meridian in two hours. The ancient astronomers measured this quantity referred to the rising point on the horizon and called it ascension.
In the RA-DEC system, now used, the great circles of HA are fixed on the celestial sphere with the zero point defined as one of the intersections of the celestial equator and the ecliptic, the vernal equinox. This coordinate is called right ascension and increases to the east from zero hours at the vernal equinox around the celestial sphere through 24 hours (360°). RA and HA are both measured in hours (h) minutes (m) and seconds (s) of time. The sky is divided into 24 hours so each hour corresponds exactly to 15° of arc (angle), each minute 15' of arc and each second 15" of arc. One degree of arc corresponds to 4 miutes of time. — Preceding unsigned comment added by 126.96.36.199 (talk) 15:20, December 16, 2004
What A Terrible Diagram
The illustrative diagram used for both RA and Dec is horrible! It falls into the category of "crummy diagram for somebody who already knows the subject, and for someone who does not: Just about useless". I mean this in the nicest possible way: It would be great if it could be replaced by one or better yet about three diagrams that serve to illustrate the point better.
First of all the declination angle looks like a vector sticking up from the equator. Second, the ecliptic plane as a heavy yellow band is visually emphasized over the equatorial plane which is the entire point of the coordinate system. — Preceding unsigned comment added by 188.8.131.52 (talk) 11:49, January 17, 2007
Rewrites in the second para .
I did an overhaul of the second paragraph, to make it flow better and to make it more accurate. RA is not a "time" as the previous version had all-but-implied; it's an angle that is customarily (but not always!) measured in units of time for the sake of convenience.
Also, I'd never heard the term Sidereal Hour Angle, which is apparently legitimate terminology in celestial navigation. If one were to confuse this with the definition of Hour Angle used in ordinary astronomy, serious confusion would result. Therefore, I added a sentence to clarify this. Jthorstensen 22:53, 29 April 2007 (UTC)
We're all new at this Prof. Thorstensen :) "Since an arc has 360 degrees, an hour of right ascension is equal to 15 degrees of arc, a single minute of right ascension equal to 15 minutes of arc, and a second of right ascension equal to 15 seconds of arc." Uh, I'm about to flunk (anything less than an 'A' is flunking) my undergraduate for nonmajors astronomy course since I'd rather learn than follow dogma. Anyway, 'Sine there are 360 degrees in the arc of a full circle, an hour of right ascension is equal to 15 degrees of arc, an arcminute is equal to 15/60 degrees of an arc... 60 seconds in an minute and 60 minutes in a day and 24 hours in 360 degrees up, over, and around the sky? arcminute qv http://en.wikipedia.org/wiki/Arcminute 184.108.40.206 05:14, 10 June 2007 (UTC)David firstname.lastname@example.org (soon to be no more...)
Give the concept meaning
I'd like to see some reference points to help understand how this really applies. What RA would galactic center have?
Is really how you measure the zero point of Right Ascension?
It seems so stone age and so difficult to measure. Do we use Stone Henge to calibrate this? The first point of Aries must point fairly precisely toward a particular star, or this system would be even more moronic than it sounds. What star is it? —Preceding unsigned comment added by 220.127.116.11 (talk) 09:22, 23 March 2009 (UTC)
- It isn't based on a particular star because the coordinates will change over time due to precession of the Earth. Thus the RA coordinates are given for a particular epoch. Cf. Galactic coordinate system.—RJH (talk) 20:42, 10 November 2009 (UTC)
- RJHall is correct, and here is (I hope) another helpful note: the vernal point is where the center of the Sun appears to cross the celestial equator at the exact moment of the March equinox. This point changes slightly every year, for exactly the reasons RJHall mentioned: precession and nutation. So you could say that on March 21 of any year, the Sun has a right ascension of approximately 0 hours. CosineKitty (talk) 22:27, 3 February 2010 (UTC)
- Shouldn't the article say that the equator crosses the ecliptic in Pisces rather than Aries? (thumbnails added to right.) JDoolin (talk) 12:39, 7 January 2013 (UTC)
Error in example?
I am wondering whether the example in the current article is correct. There one reads:
"For example, if a star with RA = 01:30:00 is at a location's meridian, then a star with RA = 20:00:00 will be in the meridian 18.5 sidereal hours later."
I see it in the following way:
After 24h the second star should return to 20:00.
1:30h is 18,5h earlier than 20:00.
It should therefore be reached after 24h -18,5h =5,5h.
Am I missing an important thing?
Sidereal hour angle
This is not the correct term, as can be verified from the references to to nautical almanacs. Star hour angle (SHA) is the correct term, and is the angle measured to the West from the first point af Aries to the star. RA is not expressed in sidereal time, but in UTC and can be converted to SHA by SHA.= 360 -RA when RA has been converted to an angle at 15 degrees per hour. Reference should also be made to the Nautical Almanacs giving the position of the first point of Aries for every second of each year with an accuracy of one tenthe of a minute of arc. Ref. Nautical Almanacs as listed. Dg 18.104.22.168 (talk) 05:04, 24 January 2013 (UTC)
Aries vs Pisces?
I notice my question wasn't addressed, and that's probably because I put my question out of order, (sorry about that) but above I posted a question about Aries vs. Pisces. Why is the zero-point of right ascension called the first point of Aries, when the zero Right Ascension point on the star maps appears in Pisces? JDoolin (talk) 02:20, 1 February 2013 (UTC)
- is this to do with it being an anachronism? It was called the first point of Aries back in the era of Astrology, some 2000+ years ago, due to precession and nutation the point of zero Right Ascension has apparently moved however anachronistically its still called 'first point of Aries'.shaidar cuebiyar (talk) 00:00, 25 February 2013 (UTC)
Hey people, all you have to do is click the Wikilink first point of Aries and it explains it right there. Is this what is supposed to be confusing? We can't explain things like that in every single occurance in every single article all over Wikipedia, it would get too cumbersome. That's the point of Wikilinks. Tfr000 (talk) 01:35, 29 April 2013 (UTC)
We have figures giving the Right Ascension of all of the planets, but not that of the Earth (even though the Earth's should actually be something like 0° or 90°, right?). However, isn't this measurement confusing, since it only measures the direction that a planet's north pole is facing exclusively in relation to the Earth's orbit? Isn't there an accurate measurement which can show what direction a planet's north pole is facing in relation to its own orbital plane? BigSteve (talk) 20:33, 14 March 2013 (UTC)
h:m:s in radians?
The table giving hours, minutes, and seconds in radians lists them as π/12, π/720, and π/43200. While years of training have taught us to always reduce fractions, in this case it seems to me that listing them in their unreduced form as 2π/24, 2π/1440, and 2π/86400 might make the entries less mysterious, thereby reducing cognitive load. Comments? Vaughan Pratt (talk) 18:28, 20 June 2015 (UTC)
- Maybe. To me 2π/86400 isn't really any more meaningful than π/43200. Radians are somewhat intuitive for distance around the circumference, but not so much for angles. Tfr000 (talk) 20:46, 21 June 2015 (UTC)
- Yes, I prefer degrees myself. I also agree that my suggested version would not be meaningful for those who did not know that 2π radians is a full turn of the Earth in one day, or that a day has 24 hours, 1440 minutes, and 86400 seconds. I had in mind those who did know those things and would therefore immediately see the rationale for those fractions instead of having to do arithmetic to verify them. Vaughan Pratt (talk) 02:32, 6 July 2015 (UTC)