Precession causes the equinox points to drift westward at a rate of As the equinox shifts, it drags the coordinate grid with it. That's why star catalogs and software programs have to be updated regularly to the latest "epoch. Most catalogs and software currently use Epoch J The next major update will happen in Learning RA and Dec.
Before computer software effortlessly plotted the paths of newly discovered comets and fast-moving asteroids, I couldn't wait to get my hands on their coordinates. I'd hand-plot the positions on a paper star atlas, then swing my scope to the spot, and thrill when I found it on my own. RA and Dec. Just input its coordinates, hit enter, and you're there. If you hear of a new comet or fast-moving asteroid, a quick check of its changing coordinates will tell you not only where it is but where it's headed, so you can plan the best time to see it.
I made friends with right ascension and declination long ago. Knowing I could drive anywhere on its invisible roadways helped me, and it'll help you too become more familiar with the night sky. Good review and I enjoy RA and Dec. Much of my observing is with a Telrad. You can convert RA and Dec. Log in to Reply. Bob King said: I'd hand-plot the positions on a paper star atlas, then swing my scope to the spot, and thrill when I found it on my own. I don't know how to take this step with my Dobsonian.
Do I need some add on protractor etc? How do I find the vernal eqinox? Not much sky watching this winter. Stew Shouldice.
Bob King Post Author. Hi Stew, I also use a Dob and have for many years. You don't need to know where the vernal equinox is if you're hand-plotting.
You get the specific RA and Dec. If it's a bright star, then that's easy, you look to see if that star is up in the sky at the time you want to view it. You can do that by using a planisphere star-wheel or a free software planetarium program like Stellarium.
If you're looking for a fainter object, you'll need a star atlas. Star atlases will always shows right ascension along the bottom and declination along the side. You locate a specific object by interpolating between those numbers. Larger atlases come with their own separate fine-gradation grids you can place over the page to really nail the position down. Does this help? Can someone show me the formula to resolve altitude and azimuth of polaris using observer latitude, longitude, elevation above sea level, UT?
It is interesting that Jean Meeus Astronomical Algorithms does not even have an index entry for polaris.
Bob, this explanation would fit nicely in the first chapter of your next book! Lately I've been figuring out how to use the northern circumpolar stars and the Sun's approximate right ascension to tell time at night. Put Polaris at the middle of your imaginary clock face, find whichever of these stars are visible, and interpolate the rest of the clock. The Sun is at 0 hours RA at the March equinox, 6 hours at the June solstice, 12 hours at the September equinox, and 18 hours at the December solstice.
Figure out how far we are between an equinox and a solstice and interpolate. Astrological signs are easier, because every astrological sign is about two hours of right ascension wide, starting with 0 h at the first point of Aries, 2 h at the start of Taurus, etc. The next step is to deduce the angle between the Sun and a convenient circumpolar star, and finally to figure out where the Sun's right ascension is on your circumpolar clock. Imagine the Sun moving below the horizon from your western horizon toward the eastern horizon, and that allows you to "see" where the Sun is at any given moment from sunset to midnight and from midnight to sunrise.
The Sun is currently about one week into the sign of Pisces, or three weeks before the March equinox, so the Sun's right ascension would be about 22 hours 30 minutes according to theskylive. In other words, the Sun is about 30 minutes west of being opposite Dubhe and Merak.
So if it's nighttime and Dubhe and Merak are east of the meridian, the Sun is west of the meridian, i. When Dubhe and Merak are transiting the meridian it's 30 minutes after midnight, and when Dubhe and Merak are west of the meridian the Sun is east of the meridian, i.
By estimating the angle between the pointer stars and the meridian and adding the equation of time , I find I can estimate the time to within 15 or 30 minutes. The current equation of time is minutes 45 seconds , so subtracting a negative number means that clock time is about 13 minutes later than local solar time. You also need to correct for your longitude east or west of your standard time zone's central meridian.
For each degree east of your central meridian your clock time is four minutes earlier than mean solar time; for each degree west of your central meridian your clock time is four minutes later than mean solar time. Here in San Francisco we are 2 degrees 26 arcminutes west of the central meridian, which means our clock time is an additional ten minutes later than mean solar time.
Thanks, Anthony! Please sir can you explain to me how I can convert RA in degree minute second to arcsecond. You can convert R. But, when I think about this a bit deeper, I start to miss something. Let's just look at my "Right Ascension" problem. I understand that Right Ascension is a longitude-like celestial coordinate that varies from hrs, taken from a reference point of the vernal equinox. More specifically, for star maps that are based on the epoch Even more specifically, the vernal equinox is a specific time and date, and location of the exact point in time, when sunrise occurred, of when the day and night were exactly equal in length.
Here's question Ipso facto, we can deduce that the sunrise time for the vernal equinox should not change very much from year to year. But it isn't!!! If you track the sunrise of the vernal equinox from year to year, it varies a lot! I am obviously missing something Great questions! The vernal equinox occurs exactly when the center of the sun crosses the celestial equator moving north.
That spot is in Pisces at the moment and very slowly moving to the west due to precession. The event is completely independent of any clock or sunrise time on Earth. It is a place Earth arrives at in its orbit. If the moment of equinox occurs at 10 a. The date of the equinox changes — it can either be March 19, 20 or 21 — due to your location if it happens on March 21 at 1 a.
Other refinements keep the day from drifting from the March slot. Let me know if this helps. Hi Bob, Thanks for your quick reply. I am determined to defeat this problem!
For question 1, a lot of star maps are based on the epoch J So, this time should be a specific occurrence, for all references to go by.
I also understand that the earth is always spinning, and time and date are always relative to where you are in the globe, but this exact point of sunrise, when day and night are equal, in the year , should be a specific location -we can even disregard what time this occurred for now.
From that location, all "longitudinal" references are based. The question becomes, how come no one says for epoch year , RA is referenced from longitude X, near such and such city? When people talk about RA, they just refer to vernal equinox, but really it would be much more helpful to say even a longitude coordinate for year Can you comment on this?
Hey Bob! But I'm not a young man anymore and it's a bit tough to get some things through my feeble brain. As I read this they both lie at 60 degrees of Dec. Am I on the right track here? Sorry to be so daft. What will be the location of observer on earth? Hi Hem, It would be somewhere along the degree circle of latitude because no time is specified.
Hi Bob, Thank you for the detailed and vivid article, but as a beginner of stargazing, I am still wondering about the specific right ascension of a star. I can imagine that the declination of a star at any time of the day is fixed, and is the corresponding angle of the arc of the great circle formed by the north celestial pole and the star, but what about the right ascension. Since the earth is turning from west to east, in the Northern Hemisphere, the sky we see should be turning "anticlockwise", and if the RA is measured in the east direction, the RA of a star should decrease by 1 h after every hour is this correct?
The last question is, how do we use our eyes to measure the declination and the right ascension of stars, or is it not measured with our eyes at all? As it may be easier to find a familiar star than the vernal equinox Thank you so much! Fantastic questions. First, know that a star's right ascension R.
They don't change with the ticking of the clock. If Vega's R. Those numbers slowly change due to Earth's precession but only after decades as far as we're concerned. At the Earth's equator, the Celestial Equator is directly overhead, and the poles are on opposite sides of the horizon. All stars are visible as they rise, culminate at the meridian, and set. As you move north, the Celestial Equator mirrors your movement, moving south the same number of degrees away from the zenith the straight-overhead point as you moved north of the equator.
So, by the time you reach Austin 30 degrees North of the Equator , the Celestial Equator has moved away from the zenith, 30 degrees to the south. What has happened to the poles, meanwhile? The Celestial Equator has moved, and has taken the poles with it. The NCP has risen 30 degrees into the sky as you moved 30 degrees north , and the SCP has sunk 30 degrees below the horizon.
Had you moved south, the opposite would have occurred as the Celestial Equator moved north of the zenith. As we move north and watch the sky, we notice that some stars circle the NCP endlessly, never getting below the horizon, and that some stars vanish from our sight, never getting a chance to rise. The stars of the first group are called circumpolar stars.
Since the NCP is 30 degrees from the horizon, it makes sense that stars close to the pole -- within 30 degrees -- will never drop below the horizon as they circle the NCP. Similarly, stars with declinations ranging from degrees SCP to degrees never rise above Austin's horizon. The southernmost stars that can be seen from Austin have declinations of about degrees although hills, buildings and haze make degrees a more practical limit.
So we can say in general:. As another example, consider an observer in Tasmania, at 40 degrees S latitude. This observer would see the celestial equator 40 degrees about 4 outstretched fists, remember to the north of the zenith. No star marks the SCP, but the stars would seem to circle a point 40 degrees above the point of due south.
This observer would never see the Big or Little Dippers, but would see the constellation Scorpius pass almost directly overhead! Just as we measure declination in degrees north or south of the Celestial Equator, so do we measure RA east of a point called the Vernal Equinox.
The Vernal Equinox is the location in the sky where the Sun can be found on the first day of Spring the day is called the Vernal Equinox as well. The Sun moves through the constellations of the Zodiac over the course of the year, but the location of the Vernal Equinox stays roughly constant, and the Sun returns there every March We typically do not measure RA in degrees, but rather in hours. The reason we do this is to assist us in timing our observations. The sky "spins" through a complete circle of degrees every 24 hours, or 15 degrees every hour.
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