# Initial determination of RA for an object

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Posted (edited)

Excuse me if this question is too elementary for this forum, or in the wrong forum - I'm a newbie here.

How do astronomers initially determine the RA of a star, for star catalogues and nautical almanacs for example? While I believe I understand RA, I don't know how it is figured to begin with. I was surprised that I can't find the specific answer to this anywhere using google.

Is it done with transit telescopes and measuring meridian crossing times? And if so, as a secondary question, how can astronomers catalogue so many millions of stars with their accurate RA and Dec? It would appear to be almost impossible to do this manually.

Edited by Andro
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Posted (edited)

In principle, it's measuring an angle in the sky, eastwards from the zero point, which is defined as where the Sun crosses the celestial equator at the March equinox (This point is called the first point of Aries). One hour of RA = 15 degrees. There's a good Sky & Telescope article online https://skyandtelescope.org/astronomy-resources/right-ascension-declination-celestial-coordinates/

Originally, RA and declination were measured using transit telescopes. If you can measure the precise time a star (or other object) crosses the meridian (is due S or N of you), you can calculate  the time elapsed since the the first point of Aries crossed your meridian and that gives you its right ascension in hours, minutes and seconds. Declination is found by measuring the angle above the horizon at transit - overhead is equal to your latitude, so your horizon is 90degrees less than that etc.

In practice, as angular measurements could be made more precisely by telescopes, RA and Dec were measured as angular distances from reliably known objects.

Photography made a huge difference. It is possible to "plate solve" an image using a "computer" (originally a human being doing the maths, and now a machine) which means calculating the RA and Dec of the corners of the image and its centre, and how far it is rotated or otherwise distorted from the RA and Dec "grid". This means you can calculate the celestial position of any object in the image simply by measuring its x,y position on the image. This is all done automatically these days for many digital astro-photographs (and is even done in real time by telescopes to ensure they can point to the desired object accurately)

if a position is needed very precisely, it is usual to take many measurements and do statistics (e.g. an average) to even out errors.

The culmination of this approach is various satellites which survey the sky. The positions they measure are not affected by the atmosphere, they can make many measurements, very precisely, based on the images they take and they do this continually. This means they can see the tiny changes in RA and Dec caused by planets orbiting some stars, or "proper motion" as the star moves through space in orbit around the galactic centre. It also means that something like the Gaia catalogue of star positions has a precision far beyond what could be imagined, even a few decades ago.

Edited by chrisecurtis
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One other thing to add is because the earth precesses (wobbles on its axis) over a 26000 year period, the coordinates given in catalogues only strictly apply at  a particular year (epoch) eg 1900,1950 or currently 2000 so the exact location (where you point your telescope) is slightly different. Planetarium programs used to point telescopes take this into account.

Cheers

Robin

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Yes @robin_astro I believe Stellarium takes this effect into account very precisely, does it not?

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On 10/06/2024 at 07:22, Andro said:

how can astronomers catalogue so many millions of stars with their accurate RA and Dec? It would appear to be almost impossible to do this manually.

Once upon a time, the great visual astronomers did do it all with accurate position and time measurements. Because the celestial sphere isn't fixed relative to the observer it really wasn't a simple job.

Nowadays it's very simple to do electronically. Over the years the positions of the stars have been refined by ground based and orbiting observatories and those positions serve as a reference grid for all "new" positions.

I have software on my computer that can make measurements of an astronomical image containing stars, within seconds it can identify the bit of sky shown I can click on a star to find it's RA and DEC, even of a new object like, say, a supernova. It might have never been seen by anyone else but my little laptop could give me precise coordinates.

Some stars have a number of designations, in reference to the various star catalogues they are listed in. For example some star designations have the prefix HIP, they are in the Hipparcos catalogue which was built using precise satellite observations.

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2 hours ago, Andro said:

Yes @robin_astro I believe Stellarium takes this effect into account very precisely, does it not?

Hi Andro,

I have written software (FITSalize) to do sub arc-second level deformation measurements using a stationary scope and plate-solving. During development, based on the algorithms in J. Meeus, Astronomical Algorithms, (Richmond (VA), 2005), I tested my software against Stellarium and soon found out that Stellarium corrects for precession and nutation, but not for aberration (the phenomenon where celestial objects exhibit an apparent motion about their true positions based on the velocity of the observer).

Precession and nutation are accurately corrected for, I could not find significant differences with my software. In order to be still able to compare the results with Stellarium I made the aberration optional.

Nicolàs

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Another fly in the ointment comes from 'the aberration of starlight,' an effect which slightly alters a star's apparent position.

Olly

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