# Astronomers use coordinate systems to plot the position of stars in the night sky

Astronomy questions and answers – Looking up into the night sky you probably wonder how ancient stargazers were able to navigate using the stars in the night sky as their guide. One of the first things ancient stargazers did to help them navigate the night sky and the surface of the Earth was to create a coordinate system to pinpoint relative positions of the stars in the night sky in relation to one another.

Looking upward into the night sky, imagine the sky above you as a sphere of infinite size, centered on the Earth. This technique works in general because distances to the stars above you is not discernible using your naked eye, so the objects you see above you in the night sky all appear to lie on a great sphere at an infinite distance in relation to you.

Modern astronomers use two coordinate systems to determine the relative positions of objects in the night sky; the altitude-azimuth coordinate system and the equatorial coordinate system. We will talk a little about both coordinate systems currently being used by modern astronomers to help them plot the positions of the objects they view in the night sky and using celestial objects you view on your “Journey to the Beginning of Space and Time” to navigate your way through the universe.

In the altitude-azimuth coordinate system, altitude indicates the number of degrees from the horizon to the object in the night sky you’re viewing and ranges from 0 degrees at the horizon to 90 degrees at the zenith above you. Modern astronomers measure azimuth along the horizon from north to east, to the point where a line passing through the object in the night sky intersects the horizon at a right angle, and azimuth varies between 0 degrees and 360 degrees. Astronomers also subdivide each degree of azimuth into 60 arcminutes and each arcminute into 60 arcseconds, which helps to further subdivide the immense distances between each degree of measurement in the night sky.

The altitude-azimuth coordinate system doesn’t take into account the rotation of the Earth, though, and astronomers have solved this problem by fixing coordinates to the celestial sphere you imagine above you in the night sky. Celestial cartographers have created “celestial globes”, similar to the globes of the Earth that cartographers have devised for centuries to show the Earth and all of its features. On these celestial globes, you’ll find terms like the celestial equator and North and South celestial poles.

In the equatorial coordinate system, astronomers use two aspects called declination and right ascension to fix a star’s position on the celestial sphere you picture above you. Declination is analogous to Earth’s latitude and represents the angle between the object you’re viewing in the night sky above you and the celestial equator. Declination varies between 0-90 degrees, North and South of the celestial equator, and is measured in degrees, arcminutes, and arcseconds while a minus sign is used to designate objects south of the celestial equator.

The lines of circles that run through the celestial poles perpendicular to the celestial equator represent the hour circle of objects in the night sky above your head and are analogous to the meridian of longitude on the Earth. In order to fix an object’s position in the celestial sphere above you, we’ll also need to set the zero point of the longitude coordinate of the object, which astronomers call the object’s right ascension. In order to accomplish this, we need an intersection point between the Earth’s equator and its orbital plane, the ecliptic. Astronomers call this intersection point the vernal equinox and the sun appear to travel through the intersection point annually around March 21, as it moves South to North crossing the celestial equator.

The angle that lies between the vernal equinox and the point where the hour circle of the celestial object in question intersects the celestial equator is the right ascension of the object you see in the night sky. Right ascension is measured in hours (h), minutes (m), and seconds (s), from west to east, and the vernal equinox is zero-hour. There are about 24 hours in each day on the Earth, so each hour of right ascension in the night sky corresponds to 15 degrees of longitude.

The movement of the Earth and the objects in the night sky above you mean the appearance of the night sky is dynamic in nature, so celestial objects will appear to circle the celestial poles as you watch the night sky. A star with a greater distance from a celestial pole than your latitude will only be visible to you during a portion of its orbit. In this case, the star will rise in the east and set in the west. Stars that are always above your horizon are circumpolar for your latitude and you’ll see these stars for their entire orbit.

The Earth’s rotation and the movement of the stars also mean the constellations in the night sky above you travel slowly westward during the year. Pinpoint a star you know well in the night sky at exactly 9 P.M. tonight. This same star will be in the exact same position in the night sky tomorrow night, only 4 minutes earlier, at 8:56 P.M. Check the time this same star is in the same position on the next night and you’ll see this occurs at 8:52 P.M.

Do a little math and you’ll verify that in one month this set up would leave the stars in the night sky 2 hours out of phase with our first positional reading in the night sky for this same star. In 3 months, generally one season, the stars in the night sky above you will have traveled a quarter of the way across the night sky. After four seasons, this would bring the star in question back to the same position in the night sky as twelve months before.

One way to estimate distances in the night sky above you and give yourself a tool to help you navigate the universe on your “Journey to the Beginning of Space and Time” is to use star pairs in the night sky as your guide. Star travelers can learn by using star pairs in the Big Dipper, for example.

Check out this astronomy site at http://astronomytonight.yolasite.com/, and then let me know what you think?

Learn how NASA astronomers are planning on detecting extraterrestrial moons orbiting distant suns https://spaceshipearth1.wordpress.com/2013/12/31/searching-for-extraterrestrial-moons/.

Take a look at the latest natural color images taken by the Cassini spacecraft https://spaceshipearth1.wordpress.com/2013/12/22/cassini-spacecraft-show-views-of-the-solar-system-in-natural-color/.

# The Earth is moving at several different velocities at this very moment

## All motion is relative according to Einstein’s theories of space and time

Astronomy answers and questions – The Earth beneath you and the night sky above you are both moving relative to each other and you, and the universe around you. The Earth not only spins counterclockwise on its axis but also orbits Sol about once every 365 spins on its axis, give or take a few minutes, in a counterclockwise direction. Speeding through space and time at an impressive 100,000 km/hr (60,000 miles/hr), around 100 times faster than a speeding bullet, faster than the launch speed for any known spacecraft and certainly faster than Superman, the Earth’s orbit isn’t a perfect circle. In fact, the distance of the Earth to Sol during its transit differs significantly at different times, due to this non-circular orbit, but on average the distance between Earth and Sol is about 150 million kilometers (93 million miles). This distance astronomers call an astronomical unit or AU, and this unit is used by astronomers as a measuring stick of sorts, only on a bigger scale than the mile or kilometer.

## Up and down has no meaning

The axis of the Earth during its orbit is also tilted about 23 1/2 degrees from the line perpendicular to the flat plane traced out by the Earth’s orbit around Sol. This flat plane astronomers call the ecliptic plane and in reality, this axis tilt has no meaning in Einstein’s spacetime and is only useful in relation to the ecliptic plane. In Einstein’s universe, the notion of tilt by itself has no meaning in spacetime, where up and down are related to away from the center of the Earth (or any body with mass) and toward the center of mass, respectively.

The Earth’s axis also continues to point in the same general direction throughout Earth’s orbit of Sol. This direction is toward Polaris, often called the North Star by travelers and navigators, and lies within 1 degree of the north celestial pole, which makes it useful for navigating on the surface of Earth. This direction closely marks the direction of due north in the night sky and the altitude of Polaris is nearly equal to the latitude of an observer on the surface of Earth. Navigators and star gazers have used these facts for thousands of years to determine direction and location on the Earth’s surface and travel from one destination to another.

The changing position of Earth during the 365 days it takes the Earth to complete one orbit also results in the night sky above your head changing nightly. Sol appears to move against a background of distant stars in the 88 constellations in the Milky Way above you. The 12 constellations along the ecliptic plane star gazers refer to as the constellations of the Zodiac, but a thirteenth constellation, Ophiuchus lies partially on the ecliptic plane, as well.

### Earth’s movements help create seasons

The combination of the rotation of the Earth on its tilted axis and orbit around Sol also helps create the seasons we experience on Spaceshipearth1. In future articles, we’ll talk about the seasons of Earth, the meaning this has for life on Earth, and how this relates to the study of the movements of the exo-planets humans have, so far, viewed during the human “Journey to the Beginning of Space and Time”.

Check out my newest astronomy site at http://astronomytonight.yolasite.com/.

Learn how NASA astronomers are planning on detecting extraterrestrial moons orbiting distant suns https://spaceshipearth1.wordpress.com/2013/12/31/searching-for-extraterrestrial-moons/.

Take a look at the latest natural color images taken by the Cassini spacecraft https://spaceshipearth1.wordpress.com/2013/12/22/cassini-spacecraft-show-views-of-the-solar-system-in-natural-color/.

# The Earth rotates on its axis each day

## Astronomy questions and answers – The Earth is constantly in motion relative to everything around it and rotates on its axis once every day and orbits Sol once per year. The Earth’s axis is defined as an imaginary line connecting the North and South poles and passing through the center of the planet. The Earth rotates west to east, viewers above the North Pole will see the Earth move counterclockwise from their view, and this is why to star gazers the Sun and stars appear to rise in the east and set in the west every day.

Looking upward at the night sky you don’t actually feel the relative motion of the Earth beneath you, despite this you’re rotating at about 1000 km/hr, depending on where you’re situated on the Earth. Standing in the exact center of the North Pole, your relative speed of rotation is much less than if you were standing on the equator, and the closer you’re to the equator, the faster the Earth beneath you is moving. Standing on the equator the Earth beneath you is rotating at about 1,670 km/hr, move half-way to the North or South Pole, and the speed of rotation of the Earth decreases significantly to about 1,275 km/hr, and once you are standing on the exact North or the South Pole the Earth isn’t rotating. The rotation of the Earth on its axis has consequences for the planet and all life existing on the spaceshipearth1. The daily rotation of the Earth on its axis creates the night and day cycle we all rely on, and this motion combined with the spaceship earth’s orbit around Sol produces the seasonal cycles we all experience during life on Earth. We’ll talk about the Earth’s daily cycle and what this means for life on Earth in future articles.

### The spinning Earth makes many things possible

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# Everything on your “Journey to the Beginning of Space and Time” is moving relative to everything else in the universe

## The solar system is moving through the Milky Way

Astronomy questions and answers – Staring upward at the night sky above you get the notion you’re stationary in the universe, but nothing could be further from the truth. The Earth beneath you is spinning on its axis at 1000 km/hr, orbiting Sol at 100,000 km/hr, the Milky Way Galaxy at 800,000 km/hr while the solar system is moving relative to the local stars at 70,000 km/hr. In fact, the universe around us could be moving through a relative space and time of some unknown kind unimaginable to the human consciousness, and we would have no way of detecting this relative motion. We are all travelers in a sense on spaceshipearth1, which is the only habitable planet we know of for humankind that exists in the universe.

### The Milky Way is moving through the universe

Everything appears to be moving relative to everything else we view as we look outward into space and time, which makes traveling through space and time a hazardous activity at the best of times. The universe you’ll experience on your “Journey to the Beginning of Space and Time” isn’t the universe you experience on Earth. The relative motions of everything in the universe mean we’ll need to explain a few things to you about the way things work in the universe. In future articles, we’ll talk about the Earth’s rotation and orbit around Sol, and how this affects the planet, we’ll explain the Earth’s motion in the Milky Way Galaxy, and the motion of our solar system in relation to the nearby stars in the night sky. This will give you a base upon which to stand as we take you further out into the cosmos to explain the relative universe you’ll experience during your journey. Toward this goal, we’ll explain the meaning of Einstein’s General and Special Relativity for your trip and the way you’ll experience things during your journey.

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Learn why astronomy binoculars are a popular choice with amateur astronomers