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In our last astronomy MaSu on Meteors: Telescopes we briefly looked at different types of telescopes and their history. However, if you have no way of measuring where your telescope is pointed and map to compare it with looking at anything beyond the Moon and the inner planets becomes pretty much a hit and miss affair.
Before we go any further there is a little point I would like to make. While on our miniscule local scale the speed of light is astronomically fast, but the universe is a phenomenally large place and on this scale the speed of light is relatively slow. As a result when we look out into space we are looking back in time and the further away the object we are looking at the further back in time we are looking. The universe is also a highly dynamic place in which everything is in a constant state of motion Depending on how fast and how distant any observed object is the less certain we can be of that object's current position. Normally, and as I will be doing in future threads when referring to an object's position it is the observed position that is being referred to.
The next thing I would like to look at is what is referred to as Magnitude or the brightness of a star in the night sky. There are two magnitude figures we need to look at:
- Apparent Magnitude: This is the apparent brightness of a star or object in the sky as viewed by an Earthly observer. It is, however, an inverse base 2.512 logarithmic scale corrected for atmospheric attenuation. Ok, that's a bit of a mouthful so I will elaborate somewhat. The system stems from the Hellenistic system of dividing up the visible stars into six bright nesses with the brightest being 1 and the faintest visible with the unaided eye being 6. Each increase in magnitude corresponds to a halving of the brightness. The original system has been both extended and revised. Basically an increase in the magnitude by one corresponds to the brightness decreasing by a factor of 2.512 with the magnitude of Vega having a brightness of zero. It depends on where you are and how much light and atmospheric pollution you are subject to but in general the dimmest star visible to the naked eye has a magnitude of between 5 and 6. You can read more on how magnitude is created by following the links at the end of this article.
- Absolute Magnitude: This is the brightness as it would appear if the object were at a standard distance of 10 parsecs (32.61 light years, 3 x 1014 km). It is therefore somewhat a measure of the energy being output by that object.
For the time being I will leave the subject of magnitude there as it will be covered in more detail in subsequent articles.
So, how do we map the sky, work out where our telescope is pointed and what we are looking at?
There are several ways of describing the position of a celestial object, but for simplicity's sake we stick to the system that is most commonly used and which I will be using in later threads on astronomy.
The Equatorial Coordinate System: This is a system that is fundamentally the same as the longitude and latitude system that is used by cartographers to draw maps and by navigators to plot their position on those maps. I will use the illustration below to explain how it works.
Celestial Sphere: The system uses an imaginary sphere (BLACK outline) that is coaxial (RED dashed centre line) and concentric with the Earth (small BLUE sphere in the centre) onto which the position of any celestial object (YELLOW star) is projected (PURPLE line).
Celestial Equator: As with the terrestrial system there is an equatorial plane (LIGHT GREEN plane) that is perpendicular to the earths axis of rotation and concentric with the earth.
Right Ascension RA: Right Ascension (MEDIUM GREEN semi circle with GREEN details) is the celestial equivalent of longitude. It measures the distance the given object is from a fixed line (SOLID RED semicircle) subscribed on our imaginary line that is perpendicular to the Celestial Equatorial plane. RA is, however different to longitude in a couple of ways.
1. RA is not measured in degrees east or west but rather hours minutes and seconds with the full circle being 24 hours.
2. The zero RA (RED semicircle) from which the RA is measured is positioned so it corresponds with the intersection of the ecliptic and the celestial equator and which is referred to as "The First Point of Ares". The ecliptic plane is the plane that is subscribed by median of the Earth's orbit around the Sun and therefore corresponds with the apparent path of the Sun throughout the year. The ecliptic is also fairly important as the planets, majority of minor planets and asteroids have orbits that are reasonably close to it. Consequently, from our point of observation on Earth they all appear reasonably close to the ecliptic. Pluto is somewhat of an exception as its orbit is not only highly exocentric bringing it closer to the Sun than Neptune for much of its orbit, but is also inclined to the ecliptic by slightly over 17°.
Declination δ: The Declination (BLUE circle and PURPLE segment) is the celestial equivalent of latitude and like latitude it is measured in degrees minutes and seconds from the celestial equator. Declinations to the north of the celestial equator are given a positive value while those south of the equatorial equator are given a negative value.
For those who reside in the northern hemisphere you can fairly easily see where the celestial axis (RED DASHED centerline) is as it passes very close to the star Polaris. If you stand exactly at the north pole Polaris will be directly above and all the stars in the night sky will appear to move along circles centered on it.
The final point that needs to be emphasized is the celestial equatorial coordinate system is theoretically fixed. As a result it will appear to rotate with both the time of day and Earth's position as we orbit around the Sun. For the time being I will leave it at that as I will cover relating the celestial coordinate system to the real world in a subsequent thread that discusses telescope mounts and tracking systems.
In the southern hemisphere it is a little more difficult as there is no star like Polaris that almost exactly corresponds with the south pole. None the less, as with the north pole, all the stars will appear to travel in circular paths around the south celestial pole.
Something that is slightly off topic but I believe is something that needs to be discussed at this point is the difference between celestial, geographic and magnetic poles. The celestial poles are theoretical points that are infinitely distant and directly above the geographic poles. The geographic poles also coincide with the points on the Earth's surface where the axis of rotation lies. The magnetic poles are somewhat different and correspond with the north and south poles of the magnetic field that is generated by the earth's core. The north magnetic pole is currently in Northern Canada on Ellesmere Island at 82.7° North 114.4° West placing it some 811 km (504 miles) from the geographic pole. Not only that but it moves about shifting its location on the Earth's surface by some 1,100 km (684 miles) during the twentieth century.
Depending on where you are on Earth the difference between true and magnetic north will vary by an angle called the Magnetic Declination. The Earth's magnetic field is also affected locally by ferrous materials in the crust etcetera. As a result maps will usually have a correction factor called the Magnetic Deviation that needs to be applied to compass bearing and takes into account the magnetic declination as well as local effects.
Now we have a method of describing the position of any object in the sky we need a map to identify the objects we are looking at. There are a plethora of maps utilizing a myriad of techniques and technologies to represent the sky, but as you must be using a computer to read this I will only deal with what is referred to as Digital Planetariums. A planetarium is normally a device that projects a representation of the night sky onto a hemispherical screen that is usually the ceiling of the planetarium. A digital planetarium is a computer program that does something similar but rather than utilizing a spherical screen it displays an image on the screen of a computer.
There are numerous commercially available, shareware and freeware digital planetariums available and many computerized telescope mounts come with one that is specifically written to interface with the mount. If you already have a telescope with such a mount as I do then you will more than likely have and be familiar with such an application. If you do not already have a digital planetarium then a search of the internet for "Digital Planetarium Software" will reveal numerous applications that can be downloaded. However, if you are pressed for time and don't want to play around finding something that is suitable, prior to obtaining my telescope and using the associated software I used a package called Hallo Northern Sky Digital Planetarium. It is a freeware program that can be downloaded from the given link and is relatively easy to configure so it represents the sky wherever you are in either hemisphere and at whatever date and time. It also has features that allow you to create time compressed animations that make the apparent path of objects like planets, asteroids etcetera.
One of the most important factors in any scientific discipline is record keeping and amateur astronomy is no exception. As with the planetarium the software that comes with many computerized mounts often has the ability to keep records of your observations. There is, however, a freeware application called AstroByte Data Logging Software that can be downloaded from the link I have provided.
Finally there is also Google Earth which has recently been expanded to include many astronomical observations as well as the Moon.
As usual you can read further on the topics I have introduced by following the links below.
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