By Alain Dussault
We saw in the preceding chapter, that the stars and the deep sky objects are localized after a coordinate system.
On Earth, a site is located by its Longitude and its Latitude. Waterloo is situated at a latitude of 45° 20' North and a longitude of 72° 31' West. The equivalent in astronomy is the Altitude, that is the angle that make an object above the horizon and the Azimuth, which is in degree and measured positively, on horizon, from the North toward the East.
But more generally, the equatorial coordinates of objects are given in Right Ascension, measured in hours, minutes and seconds (h,m,s) and in Declination, measured in degrees, arcminutes and arcseconds (°, ', "). These coordinates are represented by the Greek letter Alpha and Delta. The Earth is not a perfect sphere, the orientation of its axe of rotation in space is affected by the gradual attraction that exerted on it, the Sun, the Moon and others planets on the equatorial terrestrial "bulge". This result in a precession movement that make oscillate the axe of rotation of the Earth in a 26,000 years cycle, approximately. The precession do affect the celestial coordinates and force us to correct them in the epoch in which they are calculated. The epoch presently used is the one of year 2000. We normally change epoch every 50 years.
| Epoch | R.A. |
Dec. |
| 2000 | 18h 36.7m | +38° 46' |
| 1950 | 18h 35.2m | +38° 44' |
As we are near year 2000, there is no problem in using the coordinates of that year to orient a telescope, for example.
Universal Time (U.T.)
The universal time is the time reference, used by astronomers. It is the local time or hours read on watches at the Greenwich meridian in England. This is the Zero meridian.
The time zone on Earth are divided in portion of 15° (360 / 24). So for each 15°. in less our more from the Greenwich reference, the local time is less or more by one hour. As the time zone of EDT is the meridian 75° West, our local hour is less than 5 hours (75 / 15), compare to the local time at Greenwich.
In order that our watch show the exact hour of our time zone, we need to live exactly on the meridian,
that is at 75° West. Waterloo, in Quebec, Canada, is located on the 72.3° West.
So to derive the true local time, we need to deduct 4 minutes
per degree in less or in more. In less in our example, according
to the average longitude of 75° degree.
For Waterloo, the true local time is (75 - 72.3 = -2.7) degree, or -2.7 x 4 = -10.8 minutes, let say 11 minutes
So The exact local time in our example is 11 minutes less then the one indicated by our watch.
So to be able to position correctly our Hour Circle of our Planisphere, we must use the Solar time. If we are at the Eastern Standard Time, we used the time indicated by our watch. On the contrary, id we are at Eastern Daylight Saving Time, we must then deduct one hour from the one indicated by our watch.
In the Waterloo case, we can neglect the 11 minutes. But for those that lives near the center of a time zone, their watch will indicate an error of plus or minus 30 minutes, in comparison to their exact local time. They must then deduct the difference.
In conclusion, to convert our local time to Universal Time, for EST, we must add 5 hours from the one showed on our watch, that is (75 / 5). If we are on EDST, then we need to add only 4 hours.
In specialized magazine, the coordinates are always given for the epoch 2000 and the time in Universal Time.
The astronomers used a logarithmic scale to assign a brilliance or magnitude to a celestial object. This notation is derived according to the Greek astronomer Hipparchus, who has assigned a first magnitude to the most brilliant stars, and a sixth magnitude to the least brilliant stars visible to the naked eyes. This scale has been standardized and quantified, in such a way that a star five time more brilliant than an other, emit in reality 100 times more light to Earth.
| Object | Magnitude |
| Sun | -27 |
| Full Moon | -12 |
| Venus at maximum | -4.9 |
| Mars, Jupiter et Mercury | -2.8 |
| Sirius | ~1.5 |
| Vega, Arcturus | -0.0 |
| Saturn | 0.2 |
| Polaris | 2.0 |
| Andromeda Galaxy | 3.5 |
| Globular cluster M13 | 6.0 |
Diffuse objects, like galaxies, appears less brilliants then their official magnitudes, because the light they emit are distributed over a wider surface, instead of being form a concentrate point of light, like a star.
The smaller the number, the more the object is bright.
Pegasus
Pegasus is an easy constellation to locate by its square form in the sky. Ancients saw there a wing horse.
Andromeda is also easy to find, because its at the left of Pegasus. Its principal stars seem to make an extension to the left corner of Pegasus.
This constellation is also interesting for binoculars observers, because it contains the Great Andromeda Galaxy. A spiral galaxy which look like our own, the Milky Way. It is visible to naked eyes for young peoples. On a sky map it is identified by an ellipse and bear the Messier number M31. M32 and M110 objects are also neighbouring galaxies to Andromeda, but they are not accessible to binoculars, because of their low magnitude of 8.1.
Perseus constellation is found below the W of Cassiopeia. This constellation contains many interesting objects for the binoculars observer.
The most spectacular object is the famous Double Clusters. They are identified by NGC 869 and NGC 884, on a sky map. NGC is the abbreviation for the New General Catalogue.
You will find there too the Messier object m34, an very nice open cluster. And also the famous variable stars, Algol. Its magnitude vary form 2.1 to 3.4 every 2.867 days. It is a two system stars. One of them circle to other. It pass in front and back the other. The decreasing luminosity take about 10 hours.
The Triangulum constellation is found under the Andromeda one et is also situated above Aries. In this constellation, you will find the Messier object M33, a spiral galaxy see on the front. A blob in binoculars.
In antiquity, at the time when the zodiac was defines, the Sun was located in Aries, at the first day of spring. That is why we continue to name vernal point, the first point in Aries.
This constellation contains no objects accessible to binoculars.
Winter sky is characterize by the equatorial constellation Orion, among which the main stars formed a double inverse trapeze.
Orion is the most beautiful constellation of the sky and its easy to locate. The star Betelgeuse is a red giant very easy to see in binoculars. The three stars in the center, the belt of Orion point to the Pleiades, the Messier object m45, a very brilliant open cluster, much impressive in binoculars. The most brilliant star of Orion is named Rigel.
At the base of the center star of the belt, we found the famous Orion nebula, visible to the naked eyes, they are the Messier objects M42 and M43. This nebula is very interesting for binoculars.
This constellation is easy to find. It contains one of the brilliant star of the winter sky, the star Capella.
In this constellation, we can found with binoculars, three open clusters. They are Messier objects M36, M37 and M38. Easy enough for binoculars.
It is the only constellation to have a pair of stars, almost of the same magnitude, Castor and Pollux. from there irradiate the Geminids in december.
To find Gemini, follow the direction given by the stars Rigel and Betelgeuse of Orion. In this constellation we found the nice open cluster, Messier M35, which is near the upper right corner of this grand celestial rectangle.
Situated not far from Gemini, shine the small constellation of Cancer. A king of Y inverse letter, formed by four stars.
This constellation happen to contain the most beautiful open cluster, called the Beehive, Messier M44. there is also the Messier object M67, more difficult to see.
This constellation located under Ursa Major, contains only faint stars. There are many Messier objects in this constellation, but of magnitude of 7 to 9, so not easy to find and see in binoculars. there is M3, an open cluster. Also M51, the famous Whirlpool galaxy, M63, a spiral galaxy and M94 another spiral galaxy of magnitude 9.
This constellation is constitute of an open cluster. It is situated between Canes Venatici and Virgo.
This constellation contains a lot of Messier objects, but all difficult to locate and observe with binoculars. there are a cluster of galaxies, like M64 or Black Eye, M83, M88, M98, M99 and M100.
A very nice big open cluster, exclusively for binoculars, because of this great field, is the object Melotte 111. Easy to locate with naked eyes and a must for binoculars observers.
Next chapter, Tools for observing (Binoculars, Telescope)
Preceding Chapter, Planisphere and search by Star Hopping
Last update August 1st, 1999.