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Astronomy | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Indy's Astronomy Glossary | |
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Finding Your Way in the Cosmos | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Right Ascension (RA) is similar to longitude on the Earth, however it is numbered differently.
Longitude starts at 0 degrees at the Prime Meridian and goes to 180 degrees
West or 180 degrees East. Since the Earth is a sphere, 180 West is equal
to 180 East. Right Ascension, on the other hand, is measured in hours instead of degrees. It starts at 0 hours at the First Point of Aries and goes all the way around the inside of the celestial sphere to 24 hours which is at the same place as 0 hours. A star that has an RA of 12 hours is on the opposite side of the sky from one that is at 0 hours. Once you know a star's RA, you can use Local Sidereal Time to predict when it will be in line with your local meridian. Declination (Dec) is very much like latitude on the Earth. A Dec of 0 degrees points to the "Celestial Equator" just like a latitude of 0 is at the Earth's Equator. Similarly, a Dec of +90 degrees points to the "North Celestial Pole" and -90 degrees points to the "South Celestial Pole." In this case, the word Celestial is used to distinguish between the North Pole in the sky and the North Pole on the Earth. The Celestial Poles lie directly above the Poles on the Earth, so if you were at the North Pole, the North Celestial Pole would be directly overhead. Stars or constellations with a large negative declination appear only in the Southern sky, and stars or constellations with a large positive declination appear only in the Northern sky. Always keep in mind when navigating around the Celestial Sphere, that RA and Dec are directions, and not three dimensional positions. RA and Dec do not specify the distance in light years from earth, just the observed direction on the two dimensional surface of the "sphere" from our viewpoint. While the stars appear fixed in position for all practical purposes, the RA and Dec co-ordinates do not stay the same over a long period of time. One reason they change is because of precession. Another reason they change is because of proper motion. |
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| Precession is a slow shift of the Earth's orientation in space caused by a very regular
wobble in the earths axis. In the case of precession, the First Point of
Aries really was in Aries when it was named around 0 A.D., but because
of the slow shift in the orientation of the Earth's axis with respect to
the rest of the galaxy, the First Point of Aries has since crept into the
stars of Pisces. The wobble in the Earth's axis causes the north celestial
pole, which is currently located very close to Polaris, to move in a loop around the north ecliptic pole every 26,000 years,
ending up back where it started. This movement of the celestial pole drags
the whole celestial-coordinate system along with it, changing the whole
grid of declination and right ascension. Since right ascension and declination
are continually changing, in about 12,000 years Vega will become the north star, and Orion will be a summer constellation instead of a winter occurrence. To account
for precession and fix a star's direction on the celestial sphere correctly
in both time and location, you need to specify the date for which a right
ascension and declination apply. The current standard is equinox 2000.0, which means right ascension and declination at the moment the year 2000
begins. When right ascension and declination are given in books and atlases,
they should be accompanied with a year date such as 1950 or 2000, indicating
the year for which the direction is exactly correct. The good news is that
the amount of change due to precession is usually so slight that it is
hardly noticeable in the span of 50 years or so, and even then is mostly
noticeable only near the celestial equator. The amount of change in the
position of the Earth's axis is about 0.0138° per year, or about 0.7° in
50 years. |
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Proper Motion is the apparent change of position of an individual star on the celestial sphere due to the drift caused by the stars observed speed of movement and angular motion on the sphere. For most stars, this amount of drift is very slight and not very obvious in the span of a human lifetime, or even over 10 generations. While the stars appear generally fixed in position, like the Sun they are each moving in their own orbit in our galaxy, much the same as the continents drift imperceptibly on the face of our planet. From our viewpoint on earth, the stars we see are all wandering around very slowly in different directions on the celestial sphere, and over the course of a 100,000 years will eventually drift far enough, from our perspective, so that the familiar constellations as we know them will eventually be replaced with completely different patterns.
The apparent motions of the stars in the night sky are caused by the earth's rotation and orbit around the Sun, and the effects of precession, and not related to proper motion. While the proper motion of a star is also an apparent motion, it is the result of the star's own motion in space. Proper Motion is simply a measure of how quickly a star appears to move in the sky based on it's own motion in space. The observed speed of a stars Proper Motion is usually noted in units of arc seconds per year. A better way is to note arc seconds per year, for both Proper Motion RA and Proper Motion Dec., to define both the observed speed and the observed direction of drift on the celestial sphere . In this case be sure to note if the values are positive or negative. With this information, provided that you also account for precession, you can calculate a stars RA and Dec. over a period of years to determine the direction where the star would appear on the celestial sphere on any given date past or future. Like RA and Dec., Proper Motion RA and Proper Motion Dec. does not indicate a stars true speed, change in true distance in light years, or change in position in three dimensional space, but just the observed motion on the two dimensional surface of the "sphere" from our viewpoint. The star with the greatest known proper motion is a dim star called Barnard's Star: It is only 5.91 light years away and moving unusually fast so that it appears to move 10.3 arc seconds per year. At this rate it takes only 180 years to move the width of the diameter of the full Moon as seen from Earth. Like Bernard's Star, the majority of the stars with large proper motion tend to be rather dim, but there are many exceptions. It is relevant to note that both Alpha Centauri and Bernard's Star happen to be our closest neighbors in the Milky Way Galaxy, and therefore would be expected to exhibit larger angular motion in a shorter time period. Stars that are very far away exhibit very little motion because of their distance. Imagine two cars, both in motion at say 65MPH, but one is very close, and the other is very far away. The one that is far away appears to move very slowly, while the closer one appears go by your location much more quickly. The apparent proper motions of the stars are affected by distance in the same way. The following table example lists some of the brighter stars that exhibit large proper motion.
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| Magnitude (M) is the relative brightness of a particular star, and magnitude numbers are used to indicate a stars brightness. As the magnitude numbers get lower, the stars get brighter. Each decrease in magnitude number is roughly 2.5 times brighter than the
next higher number. Around 100 B.C. when the Greek astronomer Hipparchus divided stars into brightness classes, he called the brightest ones 1st magnitude. Those a little dimmer he designated 2nd magnitude, and so on down to the faintest ones he could see with the naked eye, 6th magnitude. When telescopes were invented in the 1600's, astronomers found that they could see even fainter stars, up to 9th magnitude. However, with the advent of the telescope, it also turned out that some of Hipparchus's 1st magnitude stars are brighter than others. In order to accommodate these stars, the scale now extends into negative numbers. Thus the brightest objects have a negative value, and as the numbers get larger or more positive, the stars get dimmer. Using this scale, and if the viewing conditions are excellent, the dimmest stars that you can generally see with the naked eye are about 5.5 to 6.0 magnitude. Binoculars will show stars as dim as 9th magnitude, and a 6" amateur telescope will go to about 13th. Large, sensitive telescopes used by professional astronomers can reach to about 29th magnitude, and the Hubbell Space Telescope so far can just reach 30th. The brightest star at night, with a magnitude of -1.44 (negative 1.44), is Sirius (Alpha Canis Majoris), commonly called "the dog star". The fifth brightest star, with a magnitude of 0.03 (positive 0.03), is Vega (Alpha Lyrae). The planets in our own solar system have a magnitude number also, For example: Venus has a magnitude of -4.4 (negative 4.4). Since generally each decrease in magnitude number is roughly 2.5 times brighter than the next higher number, and since Sirius and the planet Venus are about 3 numbers apart, it follows that the planet Venus is about 15 times brighter than Sirius [2.53=15]. Of course our own Sun is a star also, and it has a magnitude of -26.8 making it about 12 billion times brighter than Sirius and 820 million times brighter than Venus. When exploring, remember that while brightness or magnitude may be related to a stars distance from earth, it is not a positive indication of distance. Two stars that appear directly next to each other in the sky could actually be 100 or 10,000 light years apart in terms of distance from Earth. The point to understand is that because of differences in the types of stars, size, and spectral class, the brighter star may be the one that is farther away. Also remember that even in the best viewing conditions, all of the stars you can see with the naked eye are part of our own galaxy, the Milky Way. Deep sky objects outside our galaxy are too dim to see without a telescope. |
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| Identity is another way of identifying stars where stars are given lower case letters of the Greek alphabet to indicate the brightness or magnitude of the stars, along with the genitive name in Latin of the constellation they appear in. These names are known as Bayer designations, and were introduced by Johan Bayer in his star atlas, Uranometria, in 1603. Because the brightest star in many constellations is designated as Alpha, many people logically assume that Bayer intended to put the stars in the order of their magnitude. If Bayer had the technology in 1603 to measure a stars magnitude accurately enough for that, he might have done it that way. Instead, he used the 6 traditional Hipparchus magnitude classes that astronomers were using at the time. What Bayer was able to do was arrange his catalog to list all 1st magnitude stars, then all 2nd magnitude and so on. Then he started at the head of the constellation figure and designated the first star nearest the head that was a 1st magnitude star as "Alpha". He then proceed towards the foot or tail of the figure and designated the next 1st magnitude star as "Beta", and so on until all of the 1st magnitude stars were designated. Then he started at the head again with the 2nd magnitude stars, and then the 3rd magnitude, and so on. Orion is a good example of how Bayer's naming system works. He designated Alpha Orionis as the identity for Betelgeuse (0.5), the headmost 1st magnitude star in the constellation Orion. And although the 1st magnitude star Rigel (0.12) is actually the brightest star in Orion, it is at the right knee or foot of the figure so he designated it as Beta Orionis. Then he started at the head again with the second magnitude stars. Bellatrix (1.64, Gamma), Minitaka (2.23, Delta), Alnilam (1.7, Epsilon), Alnitak (2.05, Zeta), and so on to Saiph (2.06, Kappa) at the left knee or foot. As you can see, they are not in exact order of magnitide. Then back to the head again with the third magnitude stars starting with Meissa (3.54, Lambda), etc. Of these listed, only Bellatrix, the third brightest star in the constellation Orion was lucky enough to draw the corresponding alphabet sequence (Gamma) according to the actual magnitude of the star. Maybe we should count Bellatrix as one of our lucky stars. The Bayer system is more useful way of naming stars because it describes both the constellation where the star is, as well as which specific star. In addition, because for historical and cultural reasons there are often different names for the same star, it provides a simple but common frame of reference. For example, Beta Orionis is commonly known to most of us as Rigel from the Arabic word Ar-Rijl, meaning "The Foot", but it also has been called Algebar, from the Arabic word Al-jabbar meaning "The Giant". In star catalogs you may find both Rigel and Algebar listed as Beta Orionis. The does not mean that there are two beta stars in the constellation Orion, but that there are two names for the same star. If you compare the RA, Dec, and Spectral Class, you should see that they are the same. Referring to this star's identity instead of it's name is more clear because no matter what name it had in the past, or may have tomorrow, it will always be Beta Orionis. Still, there are some cases where you may find two or more different stars in the same constellation with the same Bayer designation. An example of this is Alpha Centauri, where you have a triple star system consisting of two main stars, Alpha Centauri "A" and "B" orbiting one another elliptically every 80 years to form a binary star. To the naked eye, the very bright yellow dwarf "A", and the orange dwarf "B", are too close to be able to resolve as separate stars, and so are perceived together as Rigilkent, the fourth brightest star in the night sky. The third alpha star is Proxima, a dimmer red dwarf which which possibly may be in orbit about "A" and "B" with a period of about 500,000 years or more, and is sometimes called Alpha Centauri "C". Of course the Bayer system is not without it's issues; the key issue being that there are only 24 letters in the Greek alphabet. While it works very well for the bright stars that form the basic shape of a constellation, there are a lot more stars than that. After Omega, you have to start over with Alpha2, Beta2, etc. Bayer designations are always lower case as shown in the following table. |
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| Kelvin is a unit of temperature measured with respect to absolute zero, the temperature
where molecular motion stops. The symbol for kelvin temperature is K, and
unlike °C or °F is not preceded with the word degrees or the degree symbol. 0.0 K = -273.15 °C, where K = °C + 273.15 0.0 °C = 273.15 K, where °C = K - 273.15 0.0 K = -459.67 °F, where K = (°F + 459.67) / 1.8 0.0 °F = 255.37 K, where °F = (K x 1.8) - 459.67 5800 K = 5,526.8 °C = 9,980.3 °F = Class G2 = the Sun |
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| Spectral Class is a method of classifying stars based on their color. The color of a star depends on its surface temperature, with the hottest stars having a bluish to white tint, the cooler stars having a yellow to red tint, and the coldest stars radiating in the deep red to invisible infra-red range. Stars can range in temperature from "cold" at only 2000 K (kelvin) for an extreme red giant, to 100,000 K for a star inside a planetary nebula, to over a million K for a neutron star. First of all, they are given a letter code to indicate their MKK class. Next a decimalized number from 0.00 to 9.99 to further subdivide the classes, with 0.00 being at the hot end of the class and 9.99 being at the cool end of the class. And finally they receive a Roman numeral to indicate their luminosity class, which can also be subdivided with a lower case letter. A class IIa is
more luminous than a class IIb, and so on. Double or Binary Stars might
be noted with a plus sign as in for example A2III+A4IV. Our own star, the Sun, is a medium class yellow star exhibiting weaker hydrogen spectral bands, bands of ionized metals, and bands of neutral metals, and has a surface temperature of 5800 K, so it is class G2. That means that while the core is much hotter, the average surface temperature of the Sun is a relatively cool 10,000° F. It is interesting to note that one of the nearest stars, Alpha Centauri A is very much like our own Sun. The following tables explain the differences in the different MKK Classes and Luminosity Classes. Where star spectral data is presented in my object catalog, I used the same color code scheme to highlight the spectral class of each star listed.
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| Sidereal Time is astronomical time or star time, and is slightly different from Solar Time, the time we are used to. In astronomy, in order to know exactly when a particular star or constellation will appear overhead, we are concerned with how long it takes the Earth to spin 360 degrees with respect to the "fixed" stars on the celestial sphere. In this case, the stars on the celestial sphere are so far away that the orbit of the earth around the sun doesn't have much effect on their apparent direction. Solar time is a little different in that it is concerned with how long it takes the sun to travel 360 degrees around in the sky due to Earth's rotation. The time reference we all use is based on the Solar Day, so it is very natural for non-astronomers to think that star time and a solar day are the same, just one revolution of the Earth.  As this exaggerated drawing shows, the difference between the two is a matter of about 1 extra degree in the Earth's rotation. Since the rotating Earth is also in orbit around the Sun, it moves about one degree per day (360° / 365.25 Days = 0.9856° ) along its yearly orbit around the Sun. If you start on day 1 at noon, and after the earth has rotated a full 360° to get to day 2, the solar time will be 11:56 AM, or 4 minutes short of noon because the Earth is now in a different position with respect to the Sun. The Earth actually has to physically rotate almost 361 degrees to make the Sun look like it has traveled 360 degrees around the Sky. That is why solar time is not an easy way to predict where the stars are at any given time. The added variable of the Earth's orbit around the sun combined with the way we tell time, means that an object that is directly overhead at 8:00 PM local solar time tonight, will be about one degree West at 8:00 PM the following night, and 2 degrees West at 8:00 PM the next night, and so on until 6 months from now it is on the day side of the earth at 8:00 PM, and finally a year from now it is directly overhead again at 8:00 PM. Another way to think of it; a constellation or star that is directly overhead this week at 8:00 PM, will be directly overhead a week from now at about 7:32 PM. Actually, this works out great, because if the Earth wasn't in orbit around the Sun, we would see the same stars every night of the year. There wouldn't be summer stars and winter stars. Since astronomers can't make easy use of solar time to predict celestial directions, they use a rotational period called a Sidereal Day that is 4 minutes shorter than a solar day. The shorter day eliminates the extra degree that the Earth spins every solar day (360° / 24h = 15° per hour = 0.25° rotation per minute = 4 minutes per 1° rotation). This gives astronomers a daily timescale that focuses on how long it takes the Earth to spin 360 degrees with respect to the stars, and leaves the Sun out of the equation. Instead of defining a Sidereal Day to be 23 hours and 56 minutes, Sidereal Hours, Minutes and Seconds are defined in fractions of a Solar Day. One Solar Second = 1.00273790935 Sidereal Seconds. One Solar Minute = 1.00273790935 Sidereal Minutes. One Solar Hour = 1.00273790935 Sidereal Hours. Sidereal time is always noted in 24 hour format to match RA. To make it even easier, astronomers use Local Apparent Sidereal Time (LST) adjusted to the the same right ascension (RA) on the celestial sphere that is currently crossing the local meridian (Local North/South Line). In other words, the time when an object on the celestial sphere is directly overhead at its highest point in the sky, or culmination, as viewed from your specific local longitude on earth. Once you know what the Local Sidereal Time is, you know exactly what object is on your meridian, because the Local Sidereal Time is the same as the RA of the object. In other words, local star time. For example: Orion has an RA of about 5h 34m. So when your local sidereal clock displays 05:34:00, Orion is directly on your North/South Line. Local Sidereal Time will also allow you to figure out how many degrees East or West of your local meridian you need to look to find an object. Each 15° East of your meridian corresponds to +1h RA, and each 15° West of your meridian is -1h RA. So when your local sidereal clock displays 08:00:00, and if you wanted to find the Constellation Leo which is at RA 10:00:00, you would find Leo +2h or 30° East of the local meridian. To keep track of Local Sidereal Time, astronomers use a special clock that runs about 4 minutes fast each day (1 sec every 6 min). In fact, there are nice quartz analog Sidereal wall clocks that you can buy. You just set it to Local Sidereal Time and it will tell you which constellation is on your meridian and which star charts to use through the year. By far the easiest way is to use any number of computer programs and Javascript calculators that convert UTC from your computer's clock into Greenwich Mean Sidereal Time, and then offset it to display Local Sidereal Time. The calculations are accurate to within one or two seconds per year, which is probably more accurate than your computer clock. For scripts, all you need is a web browser. This is the one I use. Here are some other ways to find your current local Sidereal Time. 1. Look it up on a web site such as the U.S. Naval Observatory. http://tycho.usno.navy.mil/sidereal.html Enter your exact longitude, and it returns your current Local Sidereal Time. You can trust a Cesium Beam atomic clock any day, but the exact accuracy depends on how busy the server is. 2. Local Sidereal Time is always equal to the right ascension of any star or constellation currently on the observer's meridian (North/South Line). It is just a matter of how accurately you are able to line up the star with the meridian. When you find yourself in the field without your computer program to show you the current sky view, just set your watch to the RA of the object directly on your local meridian, and it will be accurate enough for a couple of nights viewing. It is also a very handy trick if you don't know your exact longitude or time zone. 3. The sidereal day begins at 0 hours when the vernal equinox crosses the celestial meridian. On or about March 21, the sun is located at the vernal equinox, and will be on your local meridian at noon. By definition, the local sidereal time on March 21 is 0h at noon, local mean time (Where you are). When you get stuck on a deserted island, you can use this fact with just your watch and compass to quickly estimate what day of the year it is. If you set your watch correctly to the RA of an object on your North/South line the night before, you can figure out the local sidereal time when the sun is on your meridian the next day. Then for example: If the local sidereal time when the Sun's shadow runs North and South is 02:08, that is roughly 32° hour angle past the vernal equinox (02:08 = 15° per hour + 1° for each 4 minutes = 32°). And since each degree difference in hour angle is approximately one solar day, that makes it roughly 32 days after March 21, or about April 22. More exactly, noon, April 21. (32° x 0.9856° per day = 31.53 days, and technically 1 PM Daylight Savings time) It may not get you rescued, but at least it will give you something to do while you wait. 4. Sidereal time can also be found with an astrolabe. The astrolabe is a very ancient astronomical computer for solving problems relating to time and the position of the Sun and stars in the sky. Modern astrolabes made of plastic can be very inexpensive. The key issue is that like local sidereal clocks, they are calibrated to work in your exact location, and are not very accurate past 150 miles radius of the location they were made for. Local sidereal time is the hour angle of the vernal equinox at Aries 0° (the so-called, "First Point of Aries"). To find the current local sidereal time, the rete is rotated to the current position of the ecliptic and the local sidereal time is read directly from the position of the vernal equinox. (Just for fun) |
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| Universal Time (UT) was long known as Greenwich Mean Time (GMT). Since quoting a time
to another astronomer depends on where you happen to be, astronomers need
a standard time reference that is not dependant on location or local time
zone. This standard reference is UT, for Universal Time zone. The Universal
Time zone is 0° longitude, which is by definition, that of a line engraved
in a brass plate in the floor of the Old Royal Observatory at Greenwich,
England. UT is always stated in the 24-hour system, where noon is called
12:00, 1 p.m. is 13:00, 2 p.m. is 14:00, and so on. Midnight is called
0:00. The following table will help you to convert UT to local time, for
both standard and daylight time. To convert from UT without using the table,
subtract the number of hours in the table header for your time zone. If
the result is negative, add 24 hours but subtract one day. For example:
to convert Sat, Feb 11, 2006 @ 1:32 UT to Pacific Standard Time, subtract
8 hours, which gives you -7, plus 24, minus one day, which gives you Friday,
Feb 10, 2006 @ 17:32 PST. For convenience, the times shown in red indicate
that you use the previous day, otherwise it is the same day. |
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Light Years (ly) is the unit of measurement used to describe how far a star is from earth, or from another star. It is the distance light travels in one year. The "speed of light" ("c") is 300,000 kilometers per second (186,000 miles per second) or just under 670 million miles per hour! That makes one light year about 9.454 trillion kilometers, 5.8746 trillion miles, or 63,197.79 Astronomical Units (AU). One AU is the average distance between the Earth and the Sun which is about 92.9558 million miles, making the sun about 8.3 Light Minutes from earth. Even at the speed of light, it takes over 8 minutes for the light from the Sun to reach Earth, and about 100,000 years for the light from the Sun to reach the other side of our own galaxy, the Milky Way. The closest star system beyond our own solar system is Alpha Centauri, at 4.2 to 4.4 light years. The nearest known galaxy beyond our own Milky
Way Galaxy is the Canis Major dwarf galaxy, and is about 25,000 light years away from the solar system and 42,000
light years from the center of the Milky Way. Currently it is believed
that the Canis Major dwarf galaxy has been, and still is colliding with
the Milky Way, and has already contributed to about 1% of the mass of our
galaxy. The closest large spiral galaxy similar to ours is M31, the Andromeda Galaxy, about two million light years away. And some of the farthest and faintest
objects we can see with the Hubbell Space Telescope may be over 12 billion
light years away. Just as the light from the Sun left the Sun over eight
minutes ago, the light we see today with Hubbell may have left these distant
objects 12 billion years ago.
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| For more on constellations, stars, meteors, etc., check out... Indy's Object Catalog - Catalog Index |
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