Understanding The Sky Above

From our perspective on the surface of the Earth, looking upwards to the Heavens, and the Sky Above, everything that our senses tell us at first glance is that we are in the center of the universe — every shining pinprick of light in the cosmos appears to turn around us.

We refer to this as the Geocentric Model of the universe (also called geocentrism). However, the geocentric view is wrong — it is an illusion caused by our frame of reference located on the surface of the Earth.

Yet the geocentric model of the universe dominated for thousands of years (in part due to the importance of this model in Hebrew, Christian and Islamic religion — YHWY/God/Allah creating the Earth with his own hands which must therefore be the center of all creation).

It wasn’t until the European Renaissance that great minds like Nicolaus Copernicus, Johannes Kepler, and Galileo Galilei made significant breakthroughs in our understanding of the cosmos in their investigations, that the Geocentric Model was laid to rest. Giving birth to the Heliocentric Model and, consequently, the foundations of Modern Science.

The Celestial Sphere

When we look up at the starry sky at night (especially when we are away from the light pollution of the cities) the sky appears to be a great hollow dome. This is the same sky that every one of our ancestors going all the way back to Africa 100,000 years ago looked upon — in almost every detail.

Our perception gives the impression of a celestial dome, with each star at an equal distance from us — some brighter than others.

The top of that dome, the point directly above our head (which in more technical language is the perpendicular line to the tangent line drawn on the curve of the Earth from our specific location on the surface) is called the zenith.

Where the dome of the celestial sphere meets the Earth, is called the horizon. When we are out in the prairies, desert, or sea — any wide-open space that is mostly flat as far as the eye can see — it is easy to see the circle of the horizon in every direction around us. (In other environments such as mountainous terrain, forests, or cities this is not as clear.)

People of the ancient world — in days long before electric light pollution, GPS, and energy production outside of fire — had a far greater familiarity with the sky than we do today.

It was used to navigate by all people, determine the time of year, and the seasons for seeding and harvest, in addition to divining the will of the gods. Shepherds, travellers, farmers, merchants, sailors and soldiers would lie under that sky nightly.

If you observe the night sky for hours, you will notice that the Stars, Sun, and Moon rise on the eastern horizon each day. Over the course of a night you can watch the stars travelling westwards across the sky, to set on the western horizon.

It really does appear as if a heavenly dome is turning around the Earth. New stars come into view as they rise in the east over the course of the night. Furthermore, different stars rise on the eastern horizon at dusk over the course of a year.

The Ancient Greeks viewed the sky as a celestial sphere. Some scholars thought that there was a literal spherical dome around the Earth, made of a transparent crystalline material, with the stars embedded within it like tiny jewels [1] — or “holes to heaven” revealing the light beyond.

The stars visible around the Earth at night appear to rotate around the Earth on an axis, as if they are attached to the inside of a Celestial Sphere. However, the reality is that the Earth is turning. It revolves around on its own axis once every 24 hours, and it is the extension of the North and South poles of the Earth’s axis that creates the illusion of the poles of the celestial sphere. (Stand in a city with buildings around, or nature with trees, look upwards and spin around, and you will achieve the same optical illusion.)

Moreover, today we know that the stars are not all at a fixed distance from us. In point of fact, they are all possess different sizes and brightness, and are located at significant distances from us — many that we can see with a telescope are not stars at all, but entirely different galaxies.

(A planetarium is a special theater where a simulation of stars and planets are projected onto the interior of a white dome.)

The stars appear to maintain their positions with respect to one another because they are so far away from us — literal cosmic distance — and we are all moving in unison with respect to one another. Each of those bright lights are actually moving on their own, but the distances are so great, that their motion is not visible (except over vast timespans.)

Groupings of stars like the Big Dipper or Orion’s Belt maintain the same shape over the course of a night — year after year, century after century, and millennia after millennia.

Even objects that we know have significant motion in the night sky (over small time spans like months, days, or even a single night like the Moon, and to a lesser degree, the Planets) appear to be relatively fixed against the stars.

We use the fact that the celestial sphere turns together, as a map in the sky to track positions of objects in the sky, and when they are visible — a celestial GPS coordinate system.

Only meteors (“shooting stars”) have significant movement relative to the fixed stars. These are chunks of ice and dust — cosmic debris — that burns up in the atmosphere.

Celestial Poles and Celestial Equator

Astronomers orient themselves in the sky using a system extending the Earths axis points — the North Pole, South Pole, and Equator — onto the celestial sphere.

The Earth’s axis is the axis of its rotation, existing only because the Earth is turning. This imaginary line naturally “extends” outwards from the poles of the Earth and creates the illusion that it intersects with the celestial sphere.

The points where it intersects is called the north celestial pole and the south celestial pole. As the Earth rotates in one direction, the sky appears to turn in the opposite direction around the celestial poles.

We also project the Earth’s equator onto the sky, and call it the celestial equator. It is halfway between the poles of the planet.

Depending on where we are located on the Earth, we will have a different view of the sky, and therefore see different stars — and different apparent motion of the celestial sphere.

Furthermore, the point in the sky where the Earth’s axis points to do not appear to turn. Instead, all the stars seem to orbit these points in the sky.

If we are north of the equator, we will see the sky turn around the North Celestial Pole (which happened to point at the pole star, the “North Star” that we call Polaris). If we stood directly at the North Pole, then the north celestial pole would be at zenith directly overhead. From this perspective, the celestial equator would be the horizon.

As you watched the stars from the North Pole, none would rise or set. They would all circle around Polaris (the North Star).

It is actually a (relatively unlikely) coincidence that the North Polar axis of the Earth points towards a specific star so precisely. If you were to stand in Antarctica, at the South Pole, no stars would rise or set — just like in the north — but would all revolve around a region of the sky rather than a specific star.

At the North Pole you would see all the stars of the Northern Celestial Hemisphere down to the Celestial Equator, and from the South Pole, all the stars of the Southern Celestial Equator.

The only region of the Earth where you can see all the stars in the sky (at night specifically, and only over the course of a year) is if you live on the Equator of the Earth. The celestial equator is at your zenith, and on the north and south horizons, you would see the respective celestial poles.

Over the course of a 24-hour period at the equator, all stars are above the horizon exactly half of the time. However, only half are visible (those that are in the sky at night) because the light from the Sun bouncing around and refracting throughout our atmosphere, is so bright as to overpower the little lights in the heavens.)

However, the majority of people to don live on the equator. Consequently most people do not experience the gift of being able to see all stars in the sky (over the course of a year). For people living in the Northern Hemisphere (United States, Canada, Russia, Europe, Central Asia, Middle East, North Africa, China and Japan) the North Celestial Pole will not be on the horizon or zenith, but somewhere between.

It will appear above the northern horizon at an angular height (altitude) equal to the observers latitude.

From these latitudes, the North Star will never set. Polaris, and the stars around it, are always above the horizon — both day and night.

This part of the sky is called the circumpolar zone. For observers in the continental US and Canada, the Big Dipper, Little Dipper, and Cassiopeia are constellations (and their constituent stars) that are part of the north circumpolar zone and never set.

For an observer located at a latitude of 45° N, all stars in the southern celestial hemisphere below 45° S will be below the southern horizon, thus invisible. Likewise, for an observer at a latitude of 32° S, all stars in the northern celestial hemisphere above 32° N will be below their northern horizon.

At this point in the history of the Earth, there happens to be a star very close to the north celestial pole — the pole star, Polaris. However, the direction of the axis of the Earth wobbles in time, so this alignment will not always exist.

Nonetheless, this axial star played a significant role in the mythologies of the world. Chinese mythology viewed the pole star as the emperor, with the stars of the Big Dipper his attendants since they never moved away from the pole star, and pernennially revolved around it.

Several Native American tribes called it the “fastener of the sky”.

Rising and Setting of the Sun

Even during the day the stars remain in the sky (obviously) but the sheer magnitude of the Sun’s light in the daytime sky makes them (mostly) invisible to the naked eye (except at dawn and dusk).

When the Sun rises over the eastern horizon — because of the Earth’s rotation — sunlight is scattered by the molecules in the atmosphere, filling our sky with light.

Going back to the Sumerians (and later Babylonians) more than 4000 years ago, the ancient astronomer priests were aware that the Sun changes position each night against the background of fixed stars.

This can be seen by noting the constellation visible on the horizon just before sunrise, and just after sunset over the course of the year.

The Sun gradually moves east relative to the stars, across the celestial sphere at a rate of about 1° each day. We call this motion of the Sun a year — the time it takes for the Sun to return to the same position among the stars.

The ancients believed that this was the Sun moving around the Earth because (to be fair) this exactly what it looks like is happening.

Today, however, we know that there is an equivalent explanation for this observation that happens to be the correct interpretation: the Earth is moving around the Sun.

(You can experience the same thing walking around a campfire at night,[1] or else a streetlamp in your local park — walk around the light source and the backdrop changes as you move, while the light source remains fixed. You are the Earth moving, in this demonstration.)

The path that the Sun appears to take through the stars and around the celestial sphere each year is called the ecliptic.

The Sun rises about 4 minutes later each day (with respect to the stars) meaning that the Earth must rotate slightly more than 360 degrees to bring the Sun back into view over the horizon each day.

As the months go on, we are looking at the Sun from different positions in our orbit around the Sun — thus the backdrop of stars changes.

We trace this motion by noting the constellations behind the Sun that it moves through (which is where the Zodiacal Constellations came from).

We can either note the constellation on the eastern horizon just before sunrise or just after sunset to estimate the Suns position, or — as astronomers do [1] — note the constellation at zenith at night exactly between sunrise and sunset times to get the opposing position when the Sun is exactly 180 degrees away visible in the daytime sky on the other side of the Earth.

The ecliptic does not lie on the celestial equator, but rather is inclined at angle of about 23.5° because of the angle of the Earth’s axis of rotation.

The points on the celestial sphere — the celestial poles and celestial equator — are the result of the Earth’s rotation about its axis. The ecliptic is the result of the Earth’s orbit around the Sun, and is therefore the result of a different motion (tied to the plane of the Earth’s orbit in our solar system, which is why the celestial equator and ecliptic differ from one another).

Planets in our solar system being tilted on their axis is normal. Uranus and Pluto are tilted to such a degree that the orbit the Sun “on their side”. [1]

The inclination of the ecliptic is why the Sun moves north and south in the sky during the seasons. (At northern latitudes, the Sun is lower in the sky during winter — causing Winter — and higher in the summer — causing Summer.)

Fixed and Wandering Stars

In addition to the Sun moving across our skies, the Moon and planets visible to the eyes — Mercury, Venus, Mars, Jupiter, and Saturn — also change position day by day.

The Moon and all the planets rise and set with stars. Yet they also have independent motion which has, from time immemorial, been noticed by those concerned with the heavens.

The Sumerians and Babylonians were the first cultures to distinguish these wandering motions. More than 2000 years ago the Greeks called them planetai — “wanderer”, which is where our designation planet derives from — to distinguish these wandering stars from the fixed stars.

Today we know that the Moon and the Planets are different from one another, and that none of them are actual Stars. Nonetheless, the Greeks called all seven of them —Mercury, Venus, Mars, Jupiter, Saturn, Uranus, and the Moon — planetai thinking them to be all the same, and stars in their own right different from the fixed stars only by their wandering motion.

The Greeks also dedicated a unit of time, the week to these seven bodies that move on their own; this is why we have 7 days in a week.

The Moon is the closest celestial body to us. It also moves the fastest. Completing one trip around the sky in about 1 month (which is also where the name “month” comes from; a reference to the lunar period).

The Moon travels about 12° (24× its own diameter) through the sky each day.

The individual paths of the Moon and planets all lie close to the ecliptic (though not exactly on it) because they are all in nearly the same plane in the solar system roughly equivalent to the equator of the Sun extended in space.

They are all found in a narrow 18-degree-wide belt centered on the ecliptic, called the zodiac. (The root of the term “zodiac” is the same as “zoo” because these constellations were often likened to animals by the ancients.) [1]

The apparent motion of the planets of months and years is a combination of their motion, and the motion of Earth around the Sun too — which can get relatively complex. Indeed, understanding this was a major challenge of scientists for thousands of years.

Constellations

If we could see all the stars at once, there are about 3000 that are visible with the naked eye in the skies above the Earth’s surface.

The ancients grouped these stars by noticeable, geometric patterns, like the Big Dipper (which stands out regardless of whether you know it or not). Each culture named these patterns in honour of gods, heroes, or monsters from myth and legend, or else for objects that they resembled.

The ancient Sumerians, Babylonians, Greeks, Egyptians, Chinese, Native American and Vedic peoples all immortalized aspects of teir culture in the sky, often grouping them in unique ways to form constellations.

This was far more important in the ancient world, because the stars could be used for everything from telling time, determining the season (for planting, harvest, expected snowfall, or flood season, etc) in addition to being used to navigate on long voyages, or mark locations.

They were also used to immortalize knowledge, and the lore of their people, as a teaching tool. “Dad, what is that group of stars called again?” “That is Orion son. See that right there? That’s his belt.” “Who was Orion?” “Well, in ancient times, Orion…”

While many stars are part of constellations, many exist that are not part of any constellation. Furthermore, modern telescopes reveal millions and millions that are too faint for the eyes to see. In light of this astronomers from the early 20th century sought to establish a more formal system for organizing the sky.

Today we use the term constellation to refer to one of 88 sectors the sky has been divided into. Modern boundaries are imaginary lines running north-south and east-west so that every point in the sky falls into a specific sector.

These sectors are not all the same size. Wherever possible, modern constellations have been named after the Latin translation of an Ancient Greek star pattern that lies within it.

Some use the term asterism to refer to noticeable star patterns within a constellation (sometimes spanning several constellations). For example, the Big Dipper is an asterism in the Ursa Major (Big Bear) constellation.

It is important to note that the constellations were not meant to look like the things they were named after. In all probability, it was an act of honour, respect, reverence, and memory (in the same way that we name mountains, cities, towns, roads, and states after things they clearly look nothing like).

Astronomy Basics

What’s Your Angle?

Astronomers measure how far apart objects appear in the sky using angles.

There are 360° in a circle. Half of the dome on the sky (from horizon-to-horizon in any direction) is 180°.

Therefore, if two stars are 18° apart from one another, they are separated by a distance of 1/10th of the total sky.

To give you a reference frame for the size of a degree, the moon is about ${1}/{2} \deg$ across — half a single degree —about the width of your pinkie at arms length.

Example I — Angles in the Sky

A circle consists of 360 degrees. We can use the formula

\[ speed = \frac{distance}{time} \]

This can give us a speed in kilometers/h, degrees/hour, radians/hour, miles/hour — any unit, so long as we are consistent in our units.

Say you observe Sirius in the southern sky just below Orion’s belt, and 5h later measure its new position, seeing that it has travelled 75° in that time. How long will it take for Sirius to return to this position again?

Solution

\[ \frac{75 \deg}{5h} = \frac{360 \deg}{x hours} \]

\[ x = \frac{ 360 \deg \times 5h}{75 \deg} = 24 h \]

We can see that it will take Sirius 24 hours to return to its previous locations.

Example II — Motion of the Moon

Consider the motion of the Moon against the background of stars of the celestial sphere. If you mark the position of the Moon against the stars at one time, and then come back in 4.5 hours to note its new position, you will find that it has moved about 2.47° in that time.

Given this angular distance travelled in this time, what is its angular speed per hour? Then calculate how many hours (then days) it will take to return to the same position in the sky (one full orbit around the Earth)?

(For reference, the Moon is about 0.5 degrees in diameter. So in 4.5 hours it has moved almost 5x its diameter.)

Solution

If it travelled 2.47 degrees in 4.5 hours, we calculate

\[ \frac{2.47 \deg}{4.5 h} = \frac{x}{1h} \simeq 0.54889 \approx 0.55 deg/hour \]

We can see that the Moon travels about 0.55 degrees/hour — just over its angular size in the sky.

Now, we know that to complete one full orbit through the sky is 360 degrees. Therefore

\[ \frac{0.55 \deg}{1 h} = \frac{360 \deg}{x} \]

\[ x = \frac{360}{0.55} = 654.545454 h \approx 654.55 h = \frac{654.55h}{24h) \approx 27.3 days \]

From these calculations, we can see that the Moon takes about 27.3 days to make one full orbit around the Earth (with respect to the stars) returning to its same place in the sky.

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