RASC Calgary Centre - The Constellations (original) (raw)

Romanian translation of this web page at: http://webhostinggeeks.com/science/constellation-calgary-rmby Web Geek Science

By: Larry McNish
Page last updated November 5, 2018
(Page originally created October 21, 2005)

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| Contents A Short History of the Constellations The Constellations of the Zodiac Regarding Sizes and Positions of the Constellations Square Degrees - the Area of something on the sky Right Ascension and Declination - the Position of something on the sky List of the Constellations The Piano and the Constellations - Coincidence? | | | | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | ------------------------------------------------- |

A Short History of the Constellations

Have you ever looked at the night sky and wondered about all those stars and the patterns that some of the stars seem to make? Well, so did all the ancient cultures of the world. In trying to explain those patterns to succeeding generations (before books were commonly available), they thought of them as "connect the dots" pictures in the night sky, and made up myths to explain how they got there. Thus started the 'mythology' of the star patterns which have been handed down from generation to generation since the time of the Babylonians and ancient Greek and Roman civilizations. This mythology was not only concerned about the major star groupings that the Sun, Moon and planets seemed to wander through (the "Zodiac") but all the other patterns that could be seen from Babylon, Greece and Rome. The constellations of the "Zodiac" date from about 600 B.C.

Today, we know that these chance alignment of bright stars that seem to make patterns is just a coincidence of our place in the Milky Way Galaxy. If we could travel about The Galaxy at will, we could view these patterns from different places and different angles and see that many of the stars are not closely related at all, but stretch far out into inter-stellar space. However, from our point of view on Earth, these star patterns or "asterisms" are fairly easy to recognize.

Surrounding these bright star "asterisms" are many dimmer stars that do not form part of the pattern. Since the time of the Greek astronomer Claudius Ptolemy(c 100 to 170 AD) and his 13 volume "Almagest" which defined an original set of 48 constellations, people have been observing and charting the bright as well as the not-so-bright stars on the sky and cataloging them in books. In order to record them they were given names, then Greek letters for the next dimmer generation, and then numbers when the invention of the telescope showed many, many more stars than could be seen with the naked eye or named or lettered.

When you take all the stars in a region of the sky and group them together under one naming or numbering scheme you have a "constellation". The constellations were originally charted by astronomers of the northern hemisphere. Then with the advent of better navigation and sailing ships exploring the southern hemisphere, all the southern constellations were documented as well.

However, there was no "right" way to do this and so many ancient star charts and globes (yes - they knew the world was round) show constellations invented and drawn by the astronomers doing the work (sometimes independently, sometimes in spite of earlier works).

It wasn't until 1930 that the International Astronomical Union (IAU) resolved all these different charts into one common, standard way of grouping the multitude of known stars and published what we now call the set of "modern constellations". In 1922 they had agreed on 88 different constellations and names, each very specifically defined as a region of the sky. Due to historical significance they could not just divide the sky up into equal regions and start anew - they had to include major asterisms in certain constellations and respect the heritage of most of the night sky's star patterns. (The boundaries themselves were drawn by Eug�ne Delporte, based on coordinates for the year 1875 (!), chosen because this system had already been used by Benjamin Gould in defining boundaries for the southern constellations.) The result is a hodge-podge of different sizes and shapes, some old constellations were dropped, some new ones formed, some stars that were in one "old" constellation suddenly found themselves in a "new" one. Thanks to other historical changes one constellation - Argo Navis - was "shipwrecked" because it was too big, and now all that remains are Carina (the Keel), Vela (the Sails), Puppis (the Stern), Pyxis (the Compass), Volans (the Flying Fish), and Columba (the Dove). One other constellation (Serpens) even consists of two completely separate pieces of the sky (Serpens Caput - the head of the serpent and Serpens Cauda - the tail of the serpent) separated by Ophiuchus - The Serpent Holder. Does this mean that instead of 88 constellations there are actually "87 and two halves" ?

Nowadays, it can be difficult to make out some of the creatures the ancients saw in the patterns of the stars since many of the dimmer stars that form part of the outlines are nearly invisible from our light-polluted cities. Not only that, but a number of southern constellations were named "in honour" of something and bear no resemblance to the item it is named for at all.

It is important to note that you cannot go out and view all the constellations in one evening. For one thing, the night side of the Earth can see only about 1/2 the sky at any time. For another, for Northern hemisphere observers, the further south you look the more the planet Earth itself gets in the way preventing you from seeing the more southerly stars. Therefore in an evening's observing, you only get to see about 1/2 of 1/2 or 1/4 of the constellations in the sky. If you go out observing from month to month, the orbit of the Earth around the sun will make some constellations disappear in the West and others rise in the East over the course of weeks. Therefore, in a year's time, you can see all the constellations visible from your location.

To see the other half of the constellations, you have to travel to the other (N or S) hemisphere and spend some more weeks or months viewing those that cannot be seen from home. People living at high altitudes near the equator have the best view of both the N and S hemispheres without traveling - this is why many of the world's largest telescopes are located in Hawaii, the Canary Islands, Chile etc.

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The Constellations of the Zodiac

As mentioned above, the ancients observed that the Sun, Moon and those 5 funny wandering stars (the planets known to the ancients) seem to be constrained to a particular region of the sky, never traveling really far north or south of a wide band of stars. They divided this sky band into 12 sections based on the old constellations, each differing in width but all of them about 16° high (+8°/-8°) and called them the "Zodiac". Each division is named for the constellation situated within its limits in the 2nd century B.C.!

The name "Zodiac" is derived from the Greek, meaning "animal circle" (also related to the word "zoo"), and it comes from the fact that most of these constellations are named for animals, such as Leo, the Lion, Taurus, the Bull and Cancer, the Crab. It turns out that this band of the sky is centered on a line called the **"ecliptic"**which is the apparent path the Sun appears to take through the sky as a result of the Earth's revolution around it (actually, it is defined as the projection the Earth's orbital plane into outer space). If we could see the stars in the daytime, we would see the Sun slowly wander from one constellation of the Zodiac to the next, making one complete circle around the sky in one year. Which constellation the Sun was in had to be inferred by drawing all the constellations, then noting which was the last to set before sunrise and which was the first to rise after sunset then assuming the Sun was half way in between.

The order of the constellations of the Zodiac (as given by the apparent motion of the Sun over a year is:

(The dates vary from year to year.)

Interestingly, the "sign of the Zodiac" which has been assigned for a given month in "horoscopes" that you'll find in your daily newspaper is not where the Sun actually is that particular month. It is based on the definitions of the constellations and where the Sun would have been thousands of years ago! This changes due to the "wobble" of the Earth's axis (known as precession - like the wobble of a spinning top) which alters the direction in the sky to which the North Pole points. Owing to the "Precession of the Equinoxes" about the Ecliptic, the "First Point of Aries" (a special point on this path) changes 1° about every 70 years and currently lies in the constellation Pisces, not Aries. In about 24,000 years when the 360° precession circuit has completed, the Zodiacal signs and constellations will again coincide. Also, over thousands of years, the relative positions of all the stars change due to their own motion through the galaxy.

** Yes, the ecliptic (and the Sun) also passes through the constellation of Ophiuchus (from about November 29/30 to December 17), which is not included among the so-called signs of the Zodiac. In fact, the Sun now spends more time traversing through Ophiuchus than nearby Scorpius but you never hear anyone say "my sign is Ophiuchus (O-fE-U-kus) the Serpent Holder".

On the left is a diagram of the Sun's path across the sky from Nov 20 to Dec 20 2008.The Sun's apparent motion is right to left along the yellow and red line ending at the bright spot indicating the Sun on Dec 20.It starts in Libra (right side) passes through a small "Texas Panhandle" portion of Scorpius, crosses all the way across the bottom of Ophiuchus and ends in Sagittarius.It's easy to see that saying the Sun is in Ophiuchus is not a minuscule error.(Click on the image to the left for a larger picture.) Original image from Starry Night Pro. Notations by the author.

In addition, because the Moon and planets are often positioned either just to the north or south of the ecliptic, it allows them to sometimes appear within the boundaries of several other non-zodiacal star patterns. For instance, Saturn can officially be within the boundaries of Orion (passing across the Hunter's club). Other constellations that can be visited by the Moon and planets include Auriga (the Charioteer), Cetus (the Whale), and Sextans (the Sextant). Not to belabour the point, but given that the zodiac extends 8° north and 8° south of the ecliptic, an additional twelve modern constellations reside in part within the zodiac's belt of stars: Auriga, Canis Minor, Cetus, Corvus, Crater, Hydra, Ophiuchus, Orion, Pegasus, Scutum, Serpens and Sextans, althoughanother reference states that the planets do not actually pass through Auriga, Canis Minor, and Serpens when their actual orbits are computed.

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Regarding Sizes and Positions of the Constellations

How "big" is something that is so far away?

Common measurements like kilometers or miles make no sense when we look at the distant patterns of stars on the night sky - the celestial sphere. We need a measurement system that works when we can't go "out there" and measure it directly. The system that works the best is based on angles - how big an angle something makes relative to our eye. Angles also work horizontally, vertically or diagonally. So we use **angular measurement**to describe distances and areas on the sky.

For example, here we see several angular
measurements of the constellation Ursa Major
and the (widely) separated stars Mizar and Alcor:

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Square Degrees - the Area of something on the sky

| The surface area of a circle is given by the equation:Area = π R 2where "R" is the radius of the circle. | | The surface area of a sphere is given by the equation:Area = 4 π R 2where "R" is the radius of the sphere. | | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ | | ----------------------------------------------------------------------------------------------------------------------------------------------------------- | | But we want the Area of the sphere in "square degrees", like our angular distance measurement.The solution is not as simple as multiplying 360° by 360° - the actual answer is a lot less (though still a large number): Area of a sphere in square degrees = 4 π (57.29578) 2 = 41,252.96(where π = 3.1415927 and 57.29578 is the number of degrees in a "radian" angle)(and, yes - the number of square degrees on a sphere is not a whole number!)So, the sizes of the constellations (given in the table below) take some advanced spherical math to figure out. | | |

So, if you took a picture that covered 1° by 1° of the sky, you would require over 41,253 pictures to cover the whole sky (with each one overlapping its neighbours slightly) due to the curvature of the celestial sphere.

Since the size of the Sun and the Moon are about 1/2° (circular) you would need more than 4 times that many rectangular pictures (about 165,000) to cover the whole sky at that resolution. Its no wonder then, that star atlases such as those used by astronomers have so many very large pages of only medium-resolution sky maps.

There are 88 constellations. Therefore, each covers (on average) about 468.8 square degrees on the sky, but they come in all sizes and shapes from the smallest - Crux (The Cross) at 68 square degrees, to the largest - Hydra (The Water Snake) at a whopping 1303 square degrees (19 times bigger than Crux)!

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Right Ascension and Declination - the Position of something on the sky

On Earth, every position on land and sea can be specified by two coordinates - your Latitude (north or south of the equator) and your Longitude (east or west of the "prime meridian of longitude" which passes through Greenwich, England).

However, we cannot extend latitude and longitude to outer space because the Earth is spinning and the stars are not.

Since the stars seem "fixed" in space we need to give them their own coordinates (like latitude and longitude) but which are fixed to the stars, not the Earth.

Although a complete description of these new coordinates - Declination and**Right Ascension** - is outside the scope of this discussion on constellations, the diagram below may assist you in understanding this immensely important astronomical concept of map coordinates in space:

Every star, galaxy, nebula and constellation position can be specified by its Right Ascension and Declination.
However, there are three things that must also be taken into account:

• Things that are not points (i.e. not stars, like: constellations, galaxies and nebulas) have a discernible size and shape and we don't want to specify a whole long list of coordinates for the boundaries of the area. We pick one such coordinate pair as the centre of the object, or in the case of constellations, the location of its mathematical "centre".
• Due to the "Precession of the Equinoxes" mentioned above, the Earth's wobble causes the positions of all celestial objects to change over time. Although this is a fairly small angle each year, it builds up over time so that every 50 years we need to re-publish all the coordinates. This is called the epoch (i.e. date) of the coordinate list. You will find many old textbooks using epoch 1950 and all newer texts using epoch 2000.
• The Solar System objects (the Sun, the Moon, the other planets, asteroids, comets etc.) have their own motion across the background of stars. For all these objects their position changes daily against the fixed coordinates for the celestial sphere - so their right ascension and declination changes from day to day. ^ top

List of the Constellations

The List of the Constellations below gives:

• the name of the constellation
• the official abbreviation of the name
• one historical meaning of the name (there are often many different historical names quoted)
• the pronunciation of the name (from the RASC Charlottetown Centre's Constellation Pronunciation Guide)
• the Genitive form of the name (i.e. possessive name used in terms like "Alpha Aquarii" meaning the first lettered star in Aquarius)
• the Area (size) in square degrees (also from the RASC Charlottetown Centre's Constellation Pronunciation Guide)
• M - number of The 110 Messier objects in the constellation
• F - number of The 111 Finest NGC objects in the constellation
• C - number of The 109 Caldwell objects in the constellation
• S - number of stars on the 100 Brightest Stars list (to magnitude 2.62)
• N - number of NGC objects in the constellation. See The NGC/IC Project or The Interactive NGC Catalog Online for more information on these objects.
• Date - MM.DD - the best date for viewing (when the constellation is on or very near the meridian at midnight Standard Time) (See the notes below for other observing times.)
• V - the constellation is Visible from Calgary, Partially visible, or Not visible.
• RA low - the lowest Right Ascension in hours and minutes
• RA high - the highest Right Ascension in hours and minutes
• Dec low - the lowest Declination in degrees and minutes
• Dec high - the highest Declination in degrees and minutes
• RA mid pt - the mathematical midpoint of its Right Ascension in hours and minutes
• Dec mid pt - the mathematical midpoint of its Declination in degrees and minutes Notes:
• All coordinates are Epoch 2000.0 and have been re-computed from the original source by the author since many of the constellation lists available elsewhere on the internet contained many errors. (The constellation boundary data below was derived from Catalog VI/49 Constellation Boundary Data (Davenhall A.C., Leggett S.K. 1989) Centre de Donn�es astronomiques de Strasbourg - CDS Vizier data catalog)
• Star coordinates change over the years due to the Precession of the Equinoxes. Since the constellations "contain" the stars within their borders, the constellation borders must also change with Precession. Over 125 years have passed since the original set of coordinates was defined, so today's constellation borders no longer "line up" with nice even values of right ascension lines or declination lines.
• In the case of the two "circumpolar" constellations - Ursa Minor and Octans - the "midpoint" of RA can be anything, so 15h was chosen for Ursa Minor, and 23h was chosen for Octans.
• Serpens is shown twice - once for Serpens Caput and again for Serpens Cauda.
• Many of the Northern Hemisphere constellations, their sky charts, and many of the fine astronomical objects within them are described on the RASC Calgary Centre - Constellation of the Month project which is still underway.
• Some constellations are "circumpolar" meaning they circle the pole star and never "set" so they are visible every night of the year, and other constellations are visible for a range of months, depending on your latitude and its Declination. In addition, the constellations rise and set daily like the Sun and the planets do, so, the time of night that you are out stargazing also matters. Consult a planisphere, one of the many PC or Mac based planetarium software packages, or one of the many internet sites to see what constellations are visible on any given night.
• Observing at times other than midnight: Since there are 24 hours of Right Ascension and 12 (roughly equal) months in the year, the time a constellation is on or near the meridian changes about 2 hours each month (or roughly 1 hour every 2 weeks) as the Earth progresses in its orbit around the Sun. So, if you observe at 10 PM instead of midnight on one of the dates in the table, you will find that the constellation is East of the meridian. To see it on the meridian at 10 PM, you have to choose the same date in the next (later) month. If you observe at 8 PM then add two months. If you observe at 9 PM add 1½ months to the date given, and so on.
• Observing during Daylight Saving Months: The dates below are calculated for Standard Time. From the beginning of March till the beginning of November our clocks are set 1 hour "early" but the constellations do not "know" this. If you go out at midnight DST, you will find that the constellation is again, east of the meridian. To make up for DST - you have to add 2 weeks to the date given in the table.

  To observe at the time indicatedadd the correction shown to the date in the table
  Month of observation	DST	8 pm	9 pm	10 pm	11 pm	Midnight	1 am	2 am
  November to March	NO	+2 mo	+1½ mo	+1 mo	+2 wks	none	-2 wks	-1 mo
  April to October	YES	+2½ mo	+2 mo	+1½ mo	+1 mo	+2 wks	none	-2 wks
  Always use the top row of this table first:
  e.g. to observe Andromeda on the meridian at 9 pm go out on October 4 +1½ months = mid-November (9 pm Standard time)
  e.g. to observe Leo Minor on the meridian at 9 pm go out on February 24 +1½ months = mid April, but that's now DST so use the second row i.e. +2 months = late April (9 pm DST)
  Note: Column headings with gray backgrounds - click to sort in ascending or descending order.
  Note: The number of NGC objects in Serpens is assigned to Seprens Serpens (Caput).
  Note: Blue numbers indicate the largest number in that column.

Note: Individual constellation areas do not sum to the actual sky area of 41,252.961249419271031294671466156 square degrees due to rounding.