Astronomy Explained

Easily understood summary of important things about astronomy including how the Sun, Moon, stars, planets, and galaxies appear to move due to the Earth’s rotation. Information about the Celestial Equator, Ecliptic, eclipses, local noon, parallax, precession of the equinoxes, Cepheid Variables and binary stars.




Annular solar eclipse at sunset on the 29-4-14 taken at 5.52 PM between Tolga and Kairi. See http://tolga.info/eclipse2014/eclipse2014.html



Introduction

This is a detailed summary to help readers gain an understanding of some basic aspects of astronomy which are important to know, but not often taught in schools.   Please give me your feedback.  I would also be curious to know if you plan to use this as a teaching aid.

I am also seeking to find people interested in astronomy, especially if they plan to travel to the Atherton Tablelands, near Cairns. We can discuss and observe the stars, clusters, planets and galaxies and point to them with a laser pointer. Beginners are also welcome. We can teach them some and loan them star charts and monoculars.  Some objects can be observed through the telescope.

Although several of the examples given here represent the view from the Southern Hemisphere, these concepts also apply equally to the Northern Hemisphere.

Observation

An incredible amount of detail can be seen with a good monocular or pair of binoculars including many nebula, open star clusters and globular clusters. Five galaxies are easily seen. Changes in brightness of variable stars are also noticeable. Binoculars have an advantage over a telescope in many ways as you can see a greater field of view and observe many more targets in a period of time. This is because it is quicker to find objects and there is no setting up required. Therefore I believe it is better for those interested in astronomy to buy either a monocular or pair of binoculars before they buy a telescope.

Some stars also have unusual colours.  For example, Betelgeuse, Aldebaran and Antares have an orange-red colour
but are referred to as red stars. Arcturus is an orange star.  By comparison, Alpha Centauri appears pale yellow and Beta Centauri has a bluish white tint.  The colour of a star is an indication of its surface temperature. The red stars are estimated at 3000 to 4000 degrees Celsius, a yellowish colour is estimated at  5000 to 6000 degrees and a bluish white star has an estimated surface temperature of 10000 to 15000 degrees.

Unlike the planets, the stars remain in fixed positions relative to the Sun. The only ones that move are binary stars which are so close together that they look like one star to the naked eye. Our galaxy does rotate and galaxies do move relative to one another. However, these changes are not noticeable in a lifetime without large telescopes. Therefore, the stars are considered to remain in fixed positions and only appear to move due to the Earth's rotation and its orbit around the Sun. There is a small amount of parallax with the closest stars as explained later but this is no greater than 0.0002 degrees. This is far too small to be perceived by a small telescope.

The celestial poles and equator
 
Due to the Earth's rotation, the Sun and stars appear to rotate around the Earth’s celestial poles. Imagine if the Earth did rotate on a visible pole that extended up into the atmosphere and into deep space. The vanishing point of this pole would be the celestial pole. At the Earth’s North Pole, the 
North Celestial Pole is vertically above you, and the South Celestial Pole is vertically below you on the other side of the centre of the Earth. At the Earth's South Pole, the South Celestial Pole is vertically above you and the North Celestial Pole is vertically below you. On the Equator, due to the curvature of the Earth, the North Celestial Pole is on the horizon towards true North and the South Celestial Pole is towards true South, on the horizon. That is because the difference in the latitude between the Equator and the poles is 90, and therefore it is 90 from the vertical. Because the Earth is sperical, the angle of elevation from the horizon (altitude) to the celestial poles is therefore the same as your latitude. For example, at 30 South, the South Celestial Pole is 30 above the horizon towards the South, and the North Celestial Pole is 30 degrees below the horizon towards the North. In the below two diagrams, the line showing the direction of the celestial poles does not extend too far, so the diagram will fit on the page. Imagine if this line was infinite in length. The blue line, at 30 South, parallel to the polar line, would also be infinite in length. Both lines would converge to the same vanishing point.


celestial pole


The Celestial Equator is an imaginary line vertically above the Equator. Imagine a large ring around the Earth in deep space which is so far away that its position does not appear to move relative to the stars when traveling from one place on the Earth to another. The angle of it above the horizon will appear to move together with the stars, but this is only due to the curvature of the Earth. In the below diagram it is not drawn too far away, so the Earth can be seen and so the diagram fits on the page. On the Equator, this line passes directly above you. However, on the poles, it is on the horizon. The maximum angle below vertical that it passes, is the same as your latitude, and it is seen to the North in the Southern Hemisphere, and to the South in the Northern Hemisphere.


celestial equator


It is important to know that the Sun and stars rotate around the celestial poles, so their path across the sky can be visualised. This enables you to locate and identify stars, determine where North and South are, and can enable you to know what time it is. Because a given star rotates around the celestial pole, the angular separation between the star and the celestial pole remains constant from a fixed location on Earth. When you look South in the Southern Hemisphere, a star very close to the 
South Celestial Pole will trace a small circle around the South Celestial Pole. The Sun, and the stars which are closer to the Celestial Equator, form a larger circle and therefore rise and set in most places in the world. The three images below show three positions of the Southern Cross as it rotates around the South Celestial Pole (over about 6 hours or quarter day/rotation). The South Celestial Pole is represented by the large blue dot. Each star remains the same angular separation from the South Celestial Pole.



You can predict the path that a celestial object takes by holding one end of a piece of cotton in one hand and stretching the arm with that hand out in front of your eyes so your fingers holding the cotton are in front of the celestial pole. With your other arm stretched out, slide the fingers of your other hand along the cotton until they are in front of the celestial object you want to predict the path of. Keep the cotton tight between both your hands. While holding the end of the cotton in front of the celestial pole, rotate the other hand, (that is in front of the celestial object), in a circle by using the length of cotton as the radius. Try to keep your hands at a constant distance from your eyes. A stick or some rope could also be used. To save having to find cotton or a stick, I normally hold my elbow up in front of the celestial pole and line part my hand or arm up with the celestial object and rotate my arm by keeping my elbow lined up with the celestial pole. If a celestial object is close to the celestial pole, you could use a hand span to as the radius by keeping one finger over the celestial pole, and another one over the celestial object, and then rotate your hand. If you notice a celestial object in an unfamiliar position and you do not know what it is, you can use the technique to trace the path it will take. This may cause you to remember what it is, because you may have noticed the celestial object in these different positions many times before. The technique can also be used to trace the path of the Sun, which is very useful as it can enable you to predict what places the Sun will shine, or where shadows will be during different times of the day. It is often important to know this because some things require shade during the day, while other things require sun. The tip of your shadow will also track around the celestial poles. During the daytime these technique can help you to determine the time, and to determine where North is.


tracking


Due to the Earth's rotation, the Sun appears to scribe an arc across the sky and rotate around the 
celestial poles every 24 hours. Therefore every hour it moves 15 around the celestial poles or passes through one degree every 4 minutes. This can help you determine North without having to use a watch.

The Earth's orbit and the Ecliptic

Because the Earth orbits around the Sun every year or 365 days, the Sun appears to move almost one degree per day from West to East relative to the background stars. This is calculated by diving 360 degrees by 365 days, which gives 0.9863 per day. In order for the Sun not to rise at a later time each day, a 24 hour day is made the length of time for the Earth to rotate 360.9863 on its axis. Stars therefore rotate an average of 15.04 (360.9863/24) per hour around the 
celestial poles, which is only slightly more than the Sun. This explains why the stars appear to move from the East to the West relative to the Sun at almost 1 degree per day and why the stars appear to rise about 4 minutes earlier every day. It should be noted that 0.9863 per day is the average figure over the year, so it varies slightly throughout the year due to the Earth's elliptical orbit around the Sun. The time of local noon, (which is the time the sun is above your degree of longitude) therefore changes slightly throughout the year. The Sun rises the same number of minutes before local noon as it sets after local noon.

The Sidereal day is the time it takes the Earth to rotate
360 relative to the stars. Its length is 23 hours 56 minutes and 4 seconds.

The path that the Sun appears to move through the stars due to the Earth’s orbit around the Sun is called the Ecliptic. You could imagine it as a line as far away as the Sun going around the Earth. It is angled to the Celestial Equator, due to the tilt of the Earth axis at 23.4. This explains why the position of the Sun moves through the year between the Tropic Of Capricorn and the Tropic Of Cancer, and why there are seasons.


ecliptic


The orbit of the planets

A superior planet is one that is further from the Sun than the Earth. It is in opposition when it is on the opposite side of the Earth relative to the sun and the three bodies (sun-earth-planet) are therefore in line as shown in the below diagram. It is about at its closest point to the Earth when in opposition. Inferior Planets (Mercury and Venus) are those closer to the Sun than the Earth. They are said to be conjunction when at the point in their orbit between the Sun and the Earth.
opposition

The planets orbit around the Sun and therefore move relative to the stars. The planets orbit in almost the same plane as the Earth does around the Sun. Therefore, they pass very close to the Ecliptic path. Planets closer to the Sun move faster.

Most of the planets and asteroids in the Solar System are further from the Sun than the Earth is. When these planets get close to being behind the Sun as seen from the Earth, they will move from West to East relative to the stars. Due to the Earth's orbit, they then go into retrograde when they are near opposition. They then move westwards. However, over a course of a year, these planets and asteroids move from West to East overall. In retrograde, these planets appear to move in the opposite direction to what they are orbiting because the Earth (which is orbiting faster around the sun) is overtaking them. You can see an example of retrograde motion if you are passing a car on a highway. The car that you are passing appears to travel backwards even though it is moving forward.

The Moon's orbit

The Moon also moves across the sky in a path very close to the Ecliptic path. Twice every month it passes right over the Ecliptic path and a solar eclipse will occur at that time if the Sun is behind it when the Moon is new. A lunar eclipse will occur as the Moon crosses the Ecliptic, if the Moon is full. The two points where the Moon cross the Ecliptic are called the Nodes. The Sun passes through them twice a year as the Earth orbits around the Sun. The times of the year when this happens are called the eclipse seasons, which extend approximately 17 days before and after each node crossing. If the Moon is new or full during these times, a solar or lunar eclipse will occur somewhere on Earth. There is slightly less than 6 months between each eclipse season. Therefore, they occur slightly earlier each year.

The Celestial Sphere

The positions of celestial objects along with the celestial poles, Equator and Ecliptic are also projected on to the Celestial Sphere which is an imaginary sphere that could be visualized as being transparent and lying out in deep space.  This sphere would be like a transparent globe of the world with the Celestial Equator and celestial poles marked on it in the positions described above.  The Celestial Sphere would also contain imaginary lines to mark the celestial coordinates, and these would be very similar to the lines of latitude and longitude on a globe of the world.  Imagine if you had a very large transparent globe of the world with your head in the centre of it. The surface of the globe would be like the Celestial Sphere. The directions (North, South, East and West) would appear the same to you as on the Celestial Sphere.  The poles of the globe would appear orientated like the celestial poles, and the globe's equator would appear orientated like the Celestial Equator.  The coordinates of latitude and longitude would be orientated like the celestial coordinates.

Celestial directions

The diagram below shows that when you look South, the celestial western side is to your right for an object above the South Celestial Pole, but to the left for objects below it (between the South Celestial Pole and the horizon). For objects to the right of the South Celestial Pole, the western side is downward, and for objects on the left of the South Celestial Pole, the western side is upwards. If an arrow is pointed towards the South Celestial Pole, the arrow would be considered to point celestial South. Arrows pointing in the opposite direction or away from the South Celestial Pole would be considered to be pointing celestial North, as shown below.  Just like on a terrestrial globe of the world, the northward pointing arrows on the opposite sides of the pole appear to point in the opposite directions when the globe is viewed from a distance. However both point North.
westpole

Below is another diagram showing an example of how two stars appear to rotate around the 
South Celestial Pole that is represented by the blue dot in the centre of the diagram. Four positions are shown which would each be 6 hours apart. The brown line drawn through the two stars shows that they they point  just to the West of the South Celestial Pole. Therefore, the star closer to the Celestial South Pole would be considered to be almost celestial South of the outer star. It is only slightly West of celestial South. The outer star would always remain almost celestial North of the inner star that is closer to the pole.  A star would be considered to be exactly celestial South of another star if a line drawn through the two stars points exactly to the Celestial South Pole.  Celestial directions therefore can differ to terrestrial directions.  For example, when the stars are in position 3 below, the outer star is terrestrial South of the inner one, but it still remains Celestial North of the inner one.
westpolestar2

When you look North, the 
celestial western side is to your left for an object above the North Celestial Pole but to the right for objects below it. For objects to the right of the North Celestial Pole, the celestial western side is upward and for objects on the left of the North Celestial Pole, the celestial western side is downwards.

Celestial coordinates

The position of a celestial object can be determined by the Declination (angle North or South of the Celestial Equator) and the Right Ascension. These are known as the celestial coordinates and 
are similar to Latitude and Longitude on a map of the Earth, but Right Ascension is expressed in hours, minutes and seconds, rather than in degrees East or West as is the case with terrestrial longitude. Right Ascension is measured eastward from an arbitrary zero point in the sky where the Sun crosses the Celestial Equator during its northward motion at the vernal equinox. To find the Right Ascension, project a line between the closest celestial pole to a celestial object and the object. Then project another line from the same celestial pole to the point in the sky where the Celestial Equator and Ecliptic intersect, where the Vernal Equinox in March occurs. The angle between the two lines is the Right Ascension, where 15 degrees is one hour. 

Navigation by the stars

Polaris, or the North Star is used by many people to determine North, because this star is very close to the North Celestial Pole and therefore appears to remain stationary. However, the problem is this star is only visible in the Northern Hemisphere and is often obscured by cloud. Therefore, it is important to know others stars to determine North as described below.

A line projected through any bright star and a fixed point (among the stars) in the sky will always point to the North or South Celestial Pole. These fixed points can be determined from star charts and are worth remembering to enable you determine the position of the celestial poles. The points can be determined by measuring the angle from a nearby bright star or bisecting a line (at a certain ratio) between a bright star and a nearby fainter star. Alternatively, the angle to a line between two bright stars and a line from one of these stars to the celestial pole will always be a constant angle. A good way to find the North or South Celestial Pole is to find pairs of bright stars that lie approximately North/South, as in the examples below. The angle between the two bright stars and the meridian line (lying North/South) is then noted to enable finding the poles. Star charts can determine this from the difference in the right ascension.


Just as in the stars shown in the above diagram, the top and bottom stars on the Southern Cross point only slightly West of the South Celestial Pole. This is also the case for the two brightest stars Sirius and Canopus.

Closer groups of stars can also be used. For example, the arrow shape of the Saucepan in Orion points only slightly West of the North Celestial Pole.  The two outer or southern stars of the bowl of the Big Dipper (Ursa Major) point slightly East of the
North Celestial Pole.

A line drawn from Procyon, through the mid-point between Pollux and Caster in Gemini, points almost exactly to the North Celestial Pole. Other stars that also point almost exactly to the North Celestial Pole are the two western stars of the Great Square of Pegasus, along with Rigel in Orion and Capella in Auriga. When the line is projected back to the celestial South, these stars point almost directly to the South Celestial Pole. The celestial pole is at the intersection of these lines, and the elevation where the celestial pole must be. Learning this can be very useful to enable navigation.

Learning and identifying stars

It is a good idea to learn the names of the brightest stars first and remember where they are as that can help you locate other objects. It is also very important to learn the path they appear to move across the sky as described above, as that can help you locate them.


The directions referred to below are only approximate and also referring to the celestial directions.
A good method is to group a pair of bright stars from different constellations together.  This will also help you to navigate by the stars, as explained above. For example, Canopus in Carina is South of Sirius in Canis Major; Rigel in Orion is south of Capella in Auriga; Procyon in Canis Minor is South of Pollux in Gemini; Altair in Aquila is South East of Vega in Lyra; and Spica in Virgo is South West of Arcturus in Bootes.  The Southern Cross in the constellation of Crux is West of the stars forming a triangle in Triangulum Australe, which is West of the star Peacock in Parvo, which is West of Alpha and Beta Grus in Gruis, which is West of Achernar in Eridanus, which is West of Canopus in Carina, which is West of the False Cross in Carina & Vela, which is West of The Southern Cross.  Fomalhaut in Piscis Austrinus is North of Alpha and Beta Grus in Gruis.

Some bright stars have a nearby, dimmer star or a group of dimmer stars that have an angular separation of between about 1 and 7 degrees away from the brighter star. These are an indicator of what the brighter star is and this is a big help in identifying them when cloud or trees obscure other surrounding stars. For example, there is a fainter star (Tarazed) about 2 degrees North West of Altair, and another one (Mirzam) about 6 degrees West of Sirius. Two fainter stars form an almost equilateral triangle with the star (Sheat) on the Northwest corner of the Great Square of Pegasis.

Measuring angular separation

You can accurately estimate angles two objects make to your eye by holding your hand slightly less than 60cm (2 feet) from your eye. At a distance of 57cm, 1 cm at right angles to the line of sight is 1 . Therefore, the number of degrees can be determined from the number of centimetres between your fingers. A ruler can also be used.  At arm’s length 1 finger thickness is approximately 1, the knuckles approx. 8 and a hand span about 17.

Parallax

The distance to closer stars can be calculated by using parallax, which is calculated from the angle that the star appears to move against very distant stars and galaxies due to the orbit of the Earth around the Sun. This is shown in the below diagram. The distant stars and galaxies would have to be so far away that they would not have any parallax angle themselves.

parallax


Cepheid Variables

A number of stars that can be observed through binoculars vary in brightness. They include Cepheid Variables which are important as they enable calculating distances to galaxies. Cepheid Variables have a bright and dim cycle, which duration is proportional to their absolute magnitude, which is how bright they would appear when viewed from a standard distance of 10 parsecs or 32.6 light years. The apparent brightness (apparent magnitude) seen from Earth can therefore be used to calculate their distance. The distances to closer C
epheid Variables were calculated using parallax and this enabled working out the above ratio.  It is assumed that more distant Cepheid Variables have the same relationship between the bright and dim cycle as the nearby ones.

Precession Of The Equinoxes

The elevation (number of degrees above the horizon) and azimuth (direction) of the stars are almost exactly the same on the same date and time each year. There is only slight variation, which is corrected by the leap year. The overall angles only change very slowly, so it may be only barely noticeable in a lifetime. This is because the Earth has a slow wobble where the position of celestial poles change, as shown in the below diagram. One complete rotation of the celestial poles takes approximately 26,000 years. This causes the dates of the seasons to slowly change. In approximately 13,000 years the time of the year that summer is now experienced will then be winter. This is called the Precession Of The Equinoxes. The Sun is also not in front of the same constellations in the relating months as it was when the signs of the Zodiac were listed thousands of years ago. Hence Astrology is not credible.

precession


Reading star charts

Circular star charts have an advantage over flat ones as they are easy to orientate and have the celestial poles in the centre. You simply hold the chart up between your head and the sky, and line up the celestial pole on the map with the celestial pole in the sky. This orientates the stars roughly how you see them after you take into account the curve of the Celestial Equator on the map. A good one to use is the National Geographic chart .

Flat charts are often used to map stars close to the Celestial Equator. However, they are also used for stars close to the poles, like the Big Dipper (Ursa Major). They have an advantage over circular ones because there is less distortion for stars close to the Celestial Equator. However, they can be difficult to orientate and visualize, especially in the Southern Hemisphere for stars in the North. You need to imagine yourself lying down with your head facing North, holding the star chart upwards to orientate it. I normally find it easier to just convert the directions where the eastern side is on the left hand side, which is the opposite side to a geographic map. For example, the bottom right hand side is South East on a geographic map, but coverts to South West on a flat astronomy chart.

There are a number of useful websites such as http://www.heavens-above.com/ which inform you of many interesting objects in the night sky in your local area, including times of International Space Station passes, iridium flares, comets and asteroids. There is a live sky chart for your area that also shows the planets and positions of the Moon and Sun.

Some other useful links are on my astronomy links page.

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