What does "Right Ascension" mean? (astronomy)?

Question

I was looking at a chart for meteor showers, and under right ascension were times, do the times mean that is the time when they are most visible?

Answer 1

Right ascension is essentially the longitude on a star chart where you may look to find something in the sky among the stars.

Facing north from the northern hemisphere, when stars rise, they ascend (rise) on the right and move from east to west, hence the term right ascension.

Right ascension is the equivalent of longitude in astronomy measured eastward from the first point of Aries (astronomical prime meridian). It is generally measured in terms of time, but can also be expressed in degrees.

The astronomical prime meridian is an imaginary N-S line crossing the equator at the same point where the sun crosses the equator at the instant spring begins. This point is zero longitude. Right ascension longitude is measured starting from this point. During the course of one year, the sun moves 360° around the sky returning to this point again at the start of the following spring.

An object with right ascension 01h 02m 03s means that the object is 1 hour, 2 minutes and 3 seconds from the astronomical prime meridian in terms of earth's rotation time (at 15 degrees per hour).

The right ascension alone does not give any indication as to when the object will be visible to you. To compute the local time of visibility you need to account for your longitude, local time and sidereal time.

The sidereal time at any moment tells you how far the astronomical prime merdian is from the center of your current sky, which you need to know to determine when an object with a given RA will be visible in the sky from your viewing location.

The stars will reach the same position in the sky again in 23h 56m 56.55536s from now, or about 4 minutes earlier tommorrow rather than in 24 hours as measured by a standard clock. As a result, the constellations move 360 degrees around the sky over the year by being about 4 minutes earlier per day.

Answer 2

What quizo said. Let me add that RA is calibrated in hours for the simple reason that the Earth (and therefore the celestial sphere) rotates a full 360° in 24 hours. So 1hr of RA is equivalent to 15°.

There is what is called sidereal time. This time is calibrated according to the hour and minute of RA that is at the local meridian -- an (imaginary) line from your north to south with the highest culimation from the horizons (not necessarily the zenith). Thus the sun is on the meridian at true noon, and if you look at the suns celestial coordinates on a planetarium program (lots of free ones out there), today Monday at when the sun is on the meridian (about 1:12 pm PDT) for California its RA is 09 hourrs 37 minutes. And because it is on the meridian the local sidereal time is ALSO 09 hrs 37 min.

Sidereal time really comes into play with setting circles and goto computerized telescopes. For most applications, just consider RA and Dec as coordinates on charts. Btw, if you want to know how far south you can see in the sky, just subtract your latitude from 90°, the difference is equivalent to how far south in declination of the celestial equator that you can see (or how far north if you live in the southern hemisphere).

Answer 3

Just as every point on the surface of the earth can be assigned a unique longitude and latitude, every point in the sky can be assigned a unique celestial longitude and celestial latitude. Now, roughly speaking, the celestial longitude is called Right Ascension (RA) and the celestial latitude is called Declination.
Now, how do we assign these unique lattitude and longitudes? Consider how we do it for terrestrial latitude and longitudes. We pick a point in in Britain and decide that the longetude passing through that point will be Zero degree longitude, right? Now how do we do it for the sky? We have to pick a point in sky. It was decided to imagine an imaginary circle going round the sky concentric to terrestrial equator. They called this "Celestial equator at infinity". Now, every year as the sun passes from southren to Northren hemisphere around March 23rd, it passes through a point in the sky. This point is called Vernal equinox. And it was decided that the imaginary longitude passing from Celestial north pole to celestial south pole would be the zero degree celestial longitude. Or equvivalently, you can call it Zer hour longitude. Because emember, angle can be measured not just in degrees, but also in hours, minutes and seconds. And with respect to this longitude different longitudes are assigned, 1,2,3,....23 hours and the 24 th is equal to 0. So that completes the full circle. Here the hours and minutes of the longitudes represents the angle they make with the Zero degree longitude as measured from the center of the earth. So rember, 1 h 8 minutes 2 seconds, refers to a longitude. Not time. Now, you can measure a celestial latitude with respect to celestial equator. Just as we do it for terrestrial latitudes. So for those in Northern Hemispheres it will be say.10, 20, 30, ....degrees and for those in southren hemisphere it will be -10, - 20.......degree etc.
So in order identify the location of say meteor showers in the sky, they would have given you RA (clestial longitude) in hours and minutes. And Declination (celestial latitude) in degrees. Generally that is the case.
I think this clears matters for you. Happy star gazing !

Answer 4

These are some very good explanations and I doubt I could add anything significant to them except to point at as a matter of interest that the star, Caph, is almost on the 0 RA meridian whereas the star, Mintaka, is almost on the celestial equator at 0 declination.

The companion to RA is HA (hour angle) and a neat bit of information is to know that RA + HA = Sidereal Time.

Answer 5

quizo actually provided a good explanation of right ascension but completely failed to answer your question. Right ascension is measured in hours, minutes, and seconds. Those aren't times, but rather they are positions. Every "hour" of right ascension translates to 15 degrees. The chart you are looking at is providing coordinates of the meteor shower's radiant point.

Answer 6

RA is the celestial equivalent of terrestrial longitude. Both RA and longitude measure an east-west angle along the equator; and both measure from an arbitrary zero point on the equator. For longitude, the zero point is the Prime Meridian; for RA, the zero point is known as the vernal equinox point (also known as the First Point of Aries), which is the place in the sky where the Sun crosses the celestial equator at the March equinox.

Answer 7

Right Ascention