[removed]

[removed]

[removed]

[removed]

[removed]

The sky has a similar set of coordinates as the Earth - in fact, the coordinates have the same names: latitude and longitudes. You’re correct in that the sky appears to move above us as the Earth rotates, but we astronomers have solved this by specifying a point on the sky that we define as always having coordinates (0,0). So, from the point of view of someone on Earth, the (0,0) point moves as the sky apparently moves above us.

The choice of where we’ve decided (0,0) to be is somewhat arbitrary, but if you’re curious, it’s defined as the position of the Sun on the sky at the vernal equinox (around March 21st).

Short answer: Right ascension and Declination are just like longitude and latitude, projected onto the sky and fixed with respect to the "fixed stars".

For a longer answer, look up the International Celestial Reference System (ICRS). It's (as far as I know) the latest standard-to-end-all-standards for celestial inertial coordinate systems.

thanks, very clear explanation... but followup

The object we are studying might not move in the same repeatable orbitable pattern around our (0,0)... like say a comet or asteroid that is moving in a straight line out of any given orbit. So regardless of our "0,0" point - that will constantly be moving ... so a day or a week or a month later - how would that specific comet be addressed?

Thanks!

We use something called ephimerides, which describe how much an object moves in longitude and latitude (Right Ascension and Declination) per day etc. In other words, they describe the trajectory of a moving celestial object. The vast majority of celestial objects don’t move appreciably on the sky from one year to the next, the exceptions being solar system objects (planets, comets, asteroids) and a handful of nearby stars.

[removed]

[removed]

[removed]

Take the Earth’s lat/long grid and imagine projecting it out in an expanding bubble out toward the stars. Lock it in place so it stays fixed with the stars* rather than spinning with the Earth. This is the astronomer’s cartographic system.

For moving objects in the solar system, we describe the shape, orientation and timing of their orbits and then use some nasty math to turn that into celestial latitude and longitude.

* turns out “fixed with respect to the stars” is really complicated.

[removed]

>The choice of where we’ve decided (0,0) to be is somewhat arbitrary, but if you’re curious, it’s defined as the position of the Sun on the sky at the vernal equinox (around March 21st).

Makes sense for it to be at the equinox... do you know, historically speaking, why this was settled upon? I mean... while it's "good enough" for long stretches of time, even this changes subtly from year to year. Is there further guidance on an annual adjustment?

Yes, a full coordinate system specifies not only the location, but also the time of the equinox. This way, the location at any other time can be calculated. You can read about the precession of the equinoxes to learn more about this.

Do we need to know the time (noon, say)? Or the location (Greenwich? Equator?)? Or are they functions of each other?

The celestial coordinates are the same for everyone, by definition. But an individual observer must account for when and where on the Earth they are looking from to know where that point will be on the sky.

[removed]

The zero latitude line (usually known as zero declination) is an imaginary line across the sky that is directly above the Earth’s equator; it’s known as the celestial equator. Since that defines the zero declination (in the same way the Earth’s equator defines zero latitude), then the next thing astronomers needed to do was to choose a point along that line to define (0,0). Rather than choose an arbitrary star, which may later prove to not lie on the celestial equator with more precise measurement, they instead chose the point where the Sun’s apparent path across the sky (known as the ecliptic) crossed the celestial equator. Two non-parallel lines must cross at a point, so this made it a good choice. I believe the choice of adopting the crossing point associated with vernal equinox was arbitrary - they could have equally likely chosen the autumnal equinox.

You are correct that the exact location of where the celestial equator and ecliptic cross will change slowly over time, so now astronomers have defined precisely where (0,0) lies relative to reference celestial objects that do not move over millenia. This point is, however, almost exactly at the traditional location defined by the celestial equator and the ecliptic.

Why not the North Star? Isn’t it a fixed position to our orbital axis?

The North Star isn’t on the celestial equator, so it doesn’t have a zero declination. It’s also not exactly aligned with the poles (it’s just under 1 degree off, which is a large offset for precision astronomy).

Oh cool, thanks for the answer!

Also, FYI the North Star isn't truly fixed either as our rotation axis is processing. The Earth is more accurately like a spinning top. It's a slow change, but in a few 1000 years there won't be a North Star. Or at least it might be a different Star, not Polaris

[removed]

Isn’t it true that Polaris is a really lucky accident as there aren’t any other bright stars along the circle that the precession traces?

Edit: spooling

Why don't they just use Earth's rotation axis, or Polaris? Is that because Earth's axis wobbles slightly, and the star positions slowly change over 1000s of years?

It is very lucky, but I don't think it's the only bright star. Vega is also quite bright and was once the North Star. But there definitely aren't many bright ones along the circle.

In summary, like lat & long on the earth, fixed objects can be referenced by Equatorial coordinates via two angles called Right Ascension ( 0h 0m 0s - 23h 59m 59s ) & Declination ( +90 degrees to -90 degrees ) plus an ephemeride date for historical reference because the zero point on both axis is defined to be the vernal equinox (the point in space where the Sun appears to cross from the southern to the northern hemisphere), but because the earth precesses this position changes, the orientation of the Earth's axial tilt changes, and with it the reference grid.

There is also Ecliptic coordinates where the zero declination is the plane of the Earth's orbit around the sun, this coordinate system is useful sometimes for calculating planetary positions as this plane of the ecliptic is tilted by 23.5 degrees to the Equatorial coordinate system due to the Earth's axial tilt.

There are also Galactic coordinates where we still measure around the sky like the other two but put the approximate centre of the galaxy as our zero point in galactic longitude & orientate galactic latitude perpendicular to the plane of our host galaxy.

Finally no matter what coordinate system is in use an orbiting object in our solar system can have its position calculated for any reasonable point in time (barring interaction) with a set or elements that state things like the Semi Major Axis, longitude of the ascending node, ellipticity etc.

Wow thanks, great and detailed answer!

[removed]

Astronomers use sidereal time, which represents one revolution of the earth, roughly 23 h 56 min. No, a day is not 24 hours long. So we use coordinated AND siderial time.

The plane in which the Sun moves in the sky and the plane of Earth's equator intersect on a line that points on one end at the zero-zero point.

>The object we are studying might not move in the same repeatable orbitable pattern around our (0,0)... like say a comet or asteroid that is moving in a straight line out of any given orbit.

Comet and asteroid orbits are just the usual conic sections, most often ellipses. Straight lines would require propulsion; actually, a bona fide "space drive". Once the orbital parameters have been found out, the orbit can be extrapolated for tens, even hundreds of years with some accuracy, and assuming there are no close encounters with massive objects.

they do. Earth's north pole projected onto the sky is the North Celestial Pole, and Earth's equator is projected onto the sky to give the Celestial Equator. Where the Sun's path in the sky intersects the Celestial Equator in spring defines the zero point.