Astronomical Coordinate Systems

Liying Huang20 May 2011

Coordinate systems on the celestial sphere

•Horizontal coordinate system•Equatorial coordinate system

•Ecliptic coordinate system•Galactic coordinate system

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Equatorial coordinate system

Ecliptic coordinate system

Galactic coordinate system

Extragalactic coordinate systems

• supergalactic coordinate system

• comoving coordinates

Supergalactic coordinate system

• By convention, supergalactic latitude and supergalactic longitude are usually denoted by SGB and SGL, respectively, by analogy to b and l conventionally used for galactic coordinates. The zero point for supergalactic longitude is defined by the intersection of this plane with the galactic plane.

• The north supergalactic pole (SGB=90°) lies at galactic coordinates (l =47.37°, b =+6.32°). In the equatorial coordinate system (epoch J2000), this is approximately (RA=18.9 h, Dec=+15.7°).

• The zero point (SGB=0°, SGL=0°) lies at (l=137.37°, b=0°). In J2000 equatorial coordinates, this is approximately (2.82 h, +59.5°).

Summary Table

Comoving coordinates• While general relativity allows one to formulate the laws of physics using arbitrary coordinates, some

coordinate choices are more natural (e.g. they are easier to work with). Comoving coordinates are an example of such a natural coordinate choice. They assign constant spatial coordinate values to observers who perceive the universe as isotropic. Such observers are called "comoving" observers because they move along with the Hubble flow.

• • A comoving observer is the only observer that will perceive the universe, including the cosmic

microwave background radiation, to be isotropic. Non-comoving observers will see regions of the sky systematically blue-shifted or red-shifted. Thus isotropy, particularly isotropy of the cosmic microwave background radiation, defines a special local frame of reference called the comoving frame. The velocity of an observer relative to the local comoving frame is called the peculiar velocity of the observer.

• • Most large lumps of matter, such as galaxies, are nearly comoving, i.e., their peculiar velocities (due

to gravitational attraction) are low.• • The comoving time coordinate is the elapsed time since the Big Bang according to a clock of a

comoving observer and is a measure of cosmological time. The comoving spatial coordinates tell us where an event occurs while cosmological time tells us when an event occurs. Together, they form a complete coordinate system, giving us both the location and time of an event.

• • Space in comoving coordinates is usually referred to as being "static", as most bodies on the scale of

galaxies or larger are approximately comoving, and comoving bodies have static, unchanging comoving coordinates. So for a given pair of comoving galaxies, while the proper distance between them would have been smaller in the past and will become larger in the future due to the expansion of space, the comoving distance between them remains constant at all times.

• • The expanding Universe has an increasing scale factor which explains how constant comoving

distances are reconciled with proper distances that increase with time.

Future

• Find your way around the sky• Find the planets• Star travel• Discover• Migrate to other planet

References• Astronomical coordinate systems,

http://spider.seds.org/spider/ScholarX/coords.html#galactic Dimitri Mihalas and James Binney. Galactic Astronomy. Structure

and Kinematics. Second edition 1981, W.H. Freeman, San Francisco. ISBN 0-7167-1280-6.

F. Schmeidler, Fundamentals of Spherical Astronomy. Ch. 2 in Compendium of Practical Astronomy, by G.D. Roth (ed.), revised translation of Handbuch für Sternfreunde, 4th edition, p. 9-35, 1994, Spinger Verlag, ISBN 0-387-53596-9

• Basics of the supergalactic coordinate system: K. Fisher fisherka@csolutions.net Original 3/2006 Rev. 7/17/2006 ht

tp://fisherka.csolutionshosting.net/astronote/Structure/Supergal/Supergalacticcoords.html

• Dr. Mary Kay Hemenway on the History of Astronomy [HQ] http://www.facebook.com/video/video.php?v=185901383064

• Comoving Coordinates, wikipedia,2011 http://en.wikipedia.org/wiki/Comoving_distance

Thank you !!!