What is Right ascension: Definition and 21 Discussions

Right ascension (abbreviated RA; symbol α) is the angular distance of a particular point measured eastward along the celestial equator from the Sun at the March equinox to the (hour circle of the) point in question above the earth.
When paired with declination, these astronomical coordinates specify the location of a point on the celestial sphere in the equatorial coordinate system.
An old term, right ascension (Latin: ascensio recta) refers to the ascension, or the point on the celestial equator that rises with any celestial object as seen from Earth's equator, where the celestial equator intersects the horizon at a right angle. It contrasts with oblique ascension, the point on the celestial equator that rises with any celestial object as seen from most latitudes on Earth, where the celestial equator intersects the horizon at an oblique angle.

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  1. G

    I Needs clarification on right ascension and this example

    NVM, these are all given: Find the altitude and azimuth of the Moon in Helsinki at midnight at the beginning of 1996. The right ascension is α = 2 h 55 min 7 s = 2.9186 h and declination δ = 14◦ 42 = 14.70◦, the sidereal time is Θ = 6 h 19 min 26 s = 6.3239 h and latitude φ = 60.16◦. It's...
  2. petha1

    Astronomy homework, right ascension

    March 21 - October 21 = 7 months. So the star is only 15m 39,3 seconds behind the sun. This means that the star won't be up when the sun is down. Answer: No, the star will set only 15 minutes after the sun has set. Therefore it won't be on the sky at the specified latitude during dark hours...
  3. D

    B Is right ascension only measured once a year?

    So I'm learning some basics about astronomy...and this right ascension value is defined as being measured eastward along the celestial equator from the vernal equinox...now as I understand it, the vernal equinox only occurs once a year (the first day of spring, in the northern hemisphere), so is...
  4. nearc

    B History of RA [why is the vernal equinox used?]

    i failed to find the history of RA [right ascension] and why the vernal equinox was picked as the starting point on the web or in the forums here. does anyone know the origins of RA? picking a solstice or equinox is obvious but why the vernal? maybe because spring in the northern hemisphere...
  5. K

    I Different densities of the universe on different angles?

    Does anybody know of a resource that I could use to determine the density of the universe along a specific line of sight? (right ascension and declination)I am investigating inertial forces and looking to identify both the most and least dense directions in space and determine by how much they...
  6. Barely_Conscious

    I How would one know whether a star would be observable?

    I'd like to answer this yes or no question for a number of objects: "Is this star, at any point between these two times, going to be above the local horizon?". Say, I'm at the prime meridian at a latitude of 50 degrees, and I want to know whether, between the sidereal times of 11:00:00 and...
  7. Jimbob999

    Optimal Right Ascension Values for Star Viewing at Different Latitudes

    Homework Statement What range of values of right ascension would be best for viewing stars at a latitude of -32 degrees, 20 arcmins, 10 arcseconds on September 21st? What about an observer at 45deg22arcm11arcs? Homework Equations N/A The Attempt at a Solution On these dates the Sun would be...
  8. H

    What is the rate of motion in Right Ascension for the asteroid?

    1. Homework Statement This is an Astrometry problem. I am not sure or how exactly to proceed: Data: First off, I am unsure which data points to use. Should I use the values from right ascension or the values from the time index. 1 hour = 15 degrees 24 hours = 360 degrees Image 01...
  9. R

    How can I determine right ascension and declination of a star in the sky?

    I have a location of approximately 35 deg 14' S and 149 deg 4' E, and I have measured the position of the star Antares to be at an altitude of 27 deg 00' using an inclinometer, and direction 247 deg using a compass. Using this information can I calculate the direction of the star on the...
  10. S

    Right Ascension and Declination

    Homework Statement Can someone help me to understand this problem: You observe an object at location(longitude –16°30’E, Latitude 28°18’N) passing the meridian (azimuth=0) at 5h (am) Coordination Universal Time (UCT). The star's elevation is 43°40’ and the stellar time at Greenwich at 0h...
  11. E

    Values of Right Ascension which are best for an observer

    What values of right ascension are best for viewing by an observer at 40 degrees north latitude in January? I found this problem in the "Introduction for the Modern Astrophysics". I'm stuck with this problem because I don't know how to relate the right ascension and latitude.
  12. W

    Connection between right ascension and time

    I'm a physics major currently taking my first astro class. We're covering the basics at the moment but I am having trouble visualizing this question from our textbook. To preface this, I understand that declination is to the celestial sphere what latitude is to the Earth and RA is to the...
  13. B

    How do you calculate declination and Right Ascension from Earth Coordinates?

    Say a person is positioned here: 40.23°N and 15.89°E and was examining the night sky. How do you calculate the declination and Right Ascension from that location's coordinates? I know the RA is measured in hours up to 24 and Declination in degrees. Any ideas?
  14. S

    How does declination and right ascension relate to latitude / longetude?

    It seems to me declination lines are vertical like longitude, and coordinates is measured horizontally (but if it runs vertical than how could it be measured as 0° at the equator?), and right ascension lines run horizontal like latitude with coordinates measured vertically. However this...
  15. G

    Help with Right Ascension and Hour Angle

    I am seeking help calculating the Right Ascension. What I would like to know specifically, is if the Right Ascension can be calculated knowing the Zodiacal Longitude or Celestial Longitude of a plant, body or star. For example, for a body at 11° Pisces 57', which is 341°57' of Celestial...
  16. Philosophaie

    Right Ascension and Declination to the Horizon

    Let's just say I was on a ship in the middle of the ocean. I am at a certain Longitude and Latitude with a Right Ascension (RA0) and Declination (DEC0) looking straight up into the heavens. If I look East how many degrees (RA and DEC) difference from my initial location to the horizon? Also...
  17. Philosophaie

    Right Ascension and Declination

    How do you find Right Ascension and Declination from basic Keplerian data?
  18. Helios

    Ecliptic Longitude and Right Ascension of the Sun

    Here's a question. Where do the ecliptic longitude and the right ascension ( of the Sun )reach their maximum difference? What is the declination for these events? What calendar dates do you suppose this happens? I know the answer. I've never heard anyone make this observation except me. These...
  19. I

    Why 24 hours for right ascension?

    I'm reading Nick Strobel's astronomy notes. I should mention that it's an excellent website for an amateur like me. I can't get one thing right though. He mentions about fixed coordinates of stars measured in RA (right-ascension) and declination. He says that "Earth's rotation is broken up into...
  20. W

    UMC Time, Right Ascension and Longitude

    Where can I find a triplet of UTC Time, Right Ascension (of zenith) and Longitude, preferably with many, many safe decimal places?
  21. B

    Right Ascension and Declination

    Hi guys, This might be a stupid question and all, but I was wondering how would you know what Right Ascension and Declination coordinates are visible from a particular location? I mean I was having a look at an old almanac lying around here and couldn't really make sense of how you can see...