The Four Kings
Published on Nov 19, 2005 at 8:02 pm.
1 Comment.
Filed under skywatching, stars.
This is a blog entry about celestial navigation.Â
Since the Earth is spherical, every spot on the planet has a unique spot in the heavens that is overhead at any particular moment. Knowing the relationship, you can determine your location on Earth. This is the essence of celestial navigation. Generally, what we are interested in is determining latitude and longitude so that we can look at a map or globe to determine our whereabouts.
Latitude is the easier dimension to measure. An approximation, to within a degree, is available from looking at the North Star (if you are in the northern hemisphere). The angle of the North Star above a flat horizon is approximately equal to your latitude. A much more precise measure of latitude is available if you can determine what is directly overhead (called the zenith). The sky can be thought of as a great sphere surrounding Earth, with a sort of latitude and longitude of its own (called declination and right ascension). The celestial coordinate of declination is a match to latitude on Earth. In fact, the declination of your zenith equals your latitude. So, if you can determine exactly what is overhead, you can determine your latitude, assuming that you have sufficiently accurate star charts or tables. Since there isn’t really a Southern Star, this is basically how you might determine latitude in the southern hemisphere (actually, it is seldom done this way).Â
Longitude is another matter. While declination is linked to latitude, longitude is not really linked to right ascension. The two coordinates are basically equivalent (one for Earth and one for the heavens). As the Earth rotates, the declination of the zenith does not change. However, the right ascension of the zenith does change over the course of the rotation of the Earth. So, you would have to know the right ascension that is overhead for each moment of the day. Worse, the right ascension of the zenith at a particular time of day changes from day to day! So, there needs to be tables and charts of this data. These tables and charts would give the right ascension that is overhead at a particular time in a particular place on Earth. Now, if you are east of that location, then that right ascension will be overhead earlier (about 4 minutes earlier for each degree longitude to the east). If you are to the west, then that right ascension will be overhead later (about 4 minutes later for each degree longitude west). So, if you have the tables and charts for a particular location, and a clock that is set to the time at that location, then you can determine your longitude in terms of how far east or west of that reference point you are by observing what right ascension is on the zenith at any particular moment. Naturally, this information would be collected by astronomers. Such information was collected and tabulated at the Royal Observatory in Greenwich, England, at one time. So, longitude would be determined east or west of that point. That is why the prime meridian goes through Greenwich.Â
OK, for those of you who know the full story, you realize that I have seriously summarized it to fit in this one blog.
While what I just said is a way of determining longitude and latitude, there is another easier way of doing the same. This involves taking sightings of certain stars. It can be determined that a particular star is overhead at a particular point on the surface of the Earth at a particular time. So, if you determine that said star is 20 degrees from being on your zenith (ie, 70 degrees altitude), then you know that you are 20 degrees away from that location. So, you can draw a circle on a map of that radius. You are somewhere on that circle. Next, you sight on another star. If it is 25 degrees from the zenith, they you draw a circle of radius 25 degrees around the spot on the map that is directly under that star at that time. You are somewhere on that circle, as well as being somewhere on the other circle. Said circles will overlap in two spots. With any luck, you will have an idea which spot is the right one. A third sighting on one additional star would have a circle that simultaneously intersected the other two at only one spot. That is your location. The minimum number of sightings is two. So, what two stars do you use? There are a LOT of stars in the sky. It would be useful to have a table of data for the absolutely fewest stars required. Well, it turns out that there are four stars that historically were often used for this purpose. These stars are near the ecliptic and are spaced roughly at right angles to one another in the sky. That means that from any latitude not more than 60 or 70 degrees from the equator, you will normally be able to always see at least two of these stars. About every 6 hours or so, you have a situation where one star is visible and two others are rising or setting. Just wait an hour or so, and you’ll have two stars again. So, what are these special stars? They are Regulus (in Leo), Aldebarran (in Taurus), Fomalhaut (in Piscis Austrinus), and Antares (in Scorpius). These are known as the four Royal Navigation Stars, or sometimes the Four Kings.
-Astroprof
Astroprof’s Page » Antares on August 14, 2007 at 9:12 pm: 1
[…] With a magnitude of 1.1, Antares is one of the 20 brightest stars in the sky (being 15th or 16th, depending upon how you count the others), and is one of the brightest stars along the ecliptic. Antares, along with Aldebarran, Regulus, and Fomalhaut, is one of the four Royal stars of navigation. […]