The Celestial Sphere
When you look up at the night sky, you get the feeling that you are beneath a vast dome with all the stars seemingly at the same distance away, like pin-pricks in a velvet curtain. The stars move across the sky, rising and setting, stuck to the inside of that dome – the celestial sphere – an imaginary spherical shell around the Earth. We see its inside surface to which the stars are stuck.
Distance in Astronomy
The celestial sphere has reference points forming the basis of coordinate systems used by astronomers and navigators to locate celestial objects' positions. The sphere is of arbitrary size, with the observer or the Earth's center at its central point, no matter where the observer is located on the Earth's surface.
It makes no difference whether you see the stars' motion due to a rotating Earth with a fixed celestial sphere or vice versa. These two perspectives give the same result: celestial objects move east to west across the sky.
All the stars and their patterns - the constellations - appear to be the same distance away. That is because they are all very, very far away. Although each star varies in distance from the Earth, the differences are imperceptible. For example, the planet Mars is only a mere 34.6 million miles away when closest to the Earth, but it might as well be 500 light years away due to the lack of depth perception in the night sky.
A light year is six trillion miles. It is the distance traveled by light in the supposed vacuum of space over a year. Sunlight speeds past us at 186,000 miles per second. That is 700 million miles per hour, sixteen billion miles a day, or six trillion miles over a year. So, if Mars were at 3,000 trillion miles away (500 light years), it would make no difference to its location on the celestial sphere, although not even the biggest telescope could resolve it.
Measuring Angular Distances
The Sun and Moon appear to be of the same angular size in the sky, yet the former is 93 million miles away compared to the Moon's quarter of a million miles. That is because the Sun is 400 times bigger than the Moon.
To gauge angles across the night sky, hold your hand at arm's length and compare the angular separation of stars with your fingers' widths and spans.
The Moon subtends an angular diameter of half a degree, 0.5° on the celestial sphere. The tip of the pinkie (your little finger) at arm's length subtends an angle of one degree, 1°. Try it. The pinkie is twice the size of the full Moon.
With your fingers and arm outstretched against the sky, the distance between the pinkie and the thumb is about 25°. Watch the following informative video about using your hands to measure angles between in the night sky:
Measuring Small Angles in the Sky
Through a telescope, astronomical objects are far smaller than the full Moon's width, so astronomers use subdivisions of a degree to measure angular distances on the celestial sphere.
They use arcminutes and arcseconds. Some astronomers use the terms seconds of arc and minutes of arc.
There are sixty arcminutes in a degree and sixty arcseconds in an arcminute like the temporal hour, minutes, and seconds.
Such small angular measures are beyond the limits of the unaided eye but fall within the range of a telescope.
Compared to the apparent motion of stars caused by the Earth's rotation on its axis, an object's actual movement as it goes across the celestial sphere - as the object moves through space - is its proper motion, in arcseconds per year.
The farther away an object, the smaller is its proper motion. Think of how slow the dwarf planet Pluto moves against the background stars compared to Jupiter. Pluto takes 248 years to circle the sky, while Jupiter does the circuit in twelve years.
The stars' proper motion is much smaller than Pluto's because they are circling the Galaxy, which takes all member stars - apart from those close to the galactic core - around 230 million years.
Celestial Sphere Reference Points
Astronomers use reference points that are fixed to the ground. One is the familiar horizon, where Earth and sky meet, the boundary between the half of the sky blocked by the land and the visible sky.
The usual four cardinal points, north, south, east, and west, are located on the horizon 90° apart. 0° is due north, 90° is due east, 180° is due south, and 270° is due west. The azimuth direction is north to east, east to south, south to west, and west back to the north:
A celestial object's altitude is the angle it makes with the horizon. The zenith is the point in the sky directly overhead and is, therefore, 90° above the horizon.
The celestial meridian is a great circle passing through the zenith and connected to the north and south cardinal points.
Observers in the southern hemisphere should invert the above image and translate the zenith and all other points to the other side of the sphere.
In the northern hemisphere, observers are familiar with the Big Dipper or Plough. They are all familiar with the Pole star, Polaris, or the North Star. The stars Dubhe and Merak of the Big Dipper point to Polaris, which by chance is very close to the North Celestial Pole or NCP for short - the point in the sky around which stars appear to rotate due to the Earth's spin, so it is useful for navigation:
Unfortunately, there is no 'Polaris' equivalent in the southern hemisphere. The South Celestial Pole or SCP for short – that is the point in the sky at which southern stars appear to rotate around - is in an area of the Octans constellation where there are no bright stars.
There is, thankfully, an equivalent of the Big Dipper in the southern hemisphere. It is the bright constellation of Crux, or more commonly, the Southern Cross.
The constellation of Crux is a similar useful navigation tool like the Big Dipper for locating the SCP as the lengthwise arms of the cross point approximately to the South Celestial Pole:
Many northern observers do not realize that in the southern hemisphere, the stars rotate clockwise around the SCP. In the North, they rotate counter- or anticlockwise around the NCP:
Not many observers in the Northern Hemisphere are familiar with the southern skies, so most misguided 'flat Earthers' live north of the equator. It is incredible the number of people these days that get sucked into false teaching - even though they witness the curved shadow of the Earth as it crosses the face of the Moon during lunar eclipses, but let us not digress!
All of this shows the spherical nature of planet Earth. Regardless of the observer's location, the celestial sphere and its markers are always in the same positions relative to the observer.
The zenith is always overhead, the horizon is always level with the ground, and the cardinal points are the fixed compass positions. Observers at different locations always have different views of the night sky, and reference points change with the location.
The Celestial Equator
The celestial equator is a projection of the Earth's equator onto the celestial sphere. It is a reference line fixed in the night sky which does not move with respect to the stars, but different observers at different latitudes will see it at different positions:
Observers on the Earth's equator at latitude zero degrees witness the celestial equator directly overhead, passing through the zenith. It spans the night sky from the eastern cardinal point through the zenith to the western cardinal point.
No matter where we are on the planet, the celestial equator always intersects the eastern and western cardinal points on the horizon. The closer to the equator we are, the nearer the celestial equator comes to the zenith.
Observers at the freezing north or south poles witness the celestial equator lining up with the horizon:
Get to Know the Celestial Sphere
At any latitude, we can build an appropriate celestial sphere. If the north and south poles are extended into the night sky, they become the north and south celestial poles.
The horizon and the celestial meridian form the reference circles for giving star positions in terms of altitude and azimuth, making it easy to find them in the night sky.
As you gain experience looking at the night sky – light-polluted or not – you will identify noticeable patterns.
The Big Dipper, the Southern Cross, the Summer Triangle asterism (made up of the three bright stars Deneb, Vega, and Altair,) and the constellation of Orion are the most prominent groupings of stars. They are the most identifiable reference points in the entire night sky.
These constellations and asterisms are signposts in the night sky and used by navigators. For hundreds of years into the future, these signposts in the sky will still be there – even though stars orbit the Galactic center at an average speed of 22,000 mph, which is indicative of the vastness of the Milky Way - which itself is a mere dot in the Universe.
Groups of stars are constellations. There are 88 constellations in total, and the oldest 48 go back to the time of ancient Greece and Babylon. Today, the constellations have clearly defined boundaries. Look at any star chart. It is easy to learn the prominent constellations, and from them, it is straightforward to navigate around the night sky:
Newcomers beware: the scale of the constellations is LARGE. VERY large. This scribe, way back in the 1960s as a mere 8-year-old, spent over an hour trying to identify the three stars of Orion's belt after watching a program of the then monthly "Sky at Night" series hosted by the late amateur astronomer and TV presenter Patrick Caldwell Moore:
Once a pattern of stars is identified, all the other constellations sky are EASY to spot. With experience, the identification of patterns in the night sky becomes very easy.
The ecliptic is the imaginary path taken across the sky by the Sun – our surrogate mother star. (If you are surprised by that definition of the Sun, then you have much to learn. Do not worry, though. Unlearning falsehoods is a pleasant experience – when what are mysteries suddenly become much less mysterious. That is what this website is about - making sense of astronomical nonsense.)
The ecliptic is aligned with the plane of the Earth's orbit around the Sun. It is the path followed by the planets:
Everyone has come across the zodiacal horoscope star signs and prophetic writings of astrologers in the mainstream press. There is no point criticizing the misguided, but most non-astronomers know the names of the ecliptic's twelve constellations from the daily newspapers.
The planets and the Moon are found relatively close to or on the ecliptic, making planetary astronomy easy. So, by finding some of the main zodiacal constellations in the night sky, it becomes pretty straightforward to identify which objects are planets.
Lesson 4 in this short introductory course is about the brightness of stars. It is about the magnitude scale. That is crucial for astronomers to understand.
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