- Introduction to the Celestial Sphere and how the celestial coordinate system works
- Introduction to Sky Charts and how to use them
- Begin to identify stars and constellations

[SLIDE 1]

When we look up at the sky at night, we cannot tell how far away an object is just by looking with the naked eye, and by object, we mean anything you expect to find in the night sky such as a star, planet, or comet. We can, however, discern the direction from one object to another.

Therefore, one way of studying the sky is by assuming that all objects are equidistant (i.e. the same distance away from the Earth), and that they are all attached to the inside of an imaginary sphere that encompasses the Earth, demonstrated in the image above. We refer to this imaginary sphere as the Celestial Sphere.

The Celestial Sphere is a useful tool in spherical astronomy that allows us to map the night sky without knowing an object's distance from Earth.


[SLIDE 2]

As we can also see from the image above, the celestial sphere not only encompasses the Earth but is also centered around it. This sphere has an infinite radius, meaning any point within the sphere can be considered the center. For example, based on the Celestial Sphere, if we take two people, one observing in Iceland and the other in New Zealand, they both can be at the center of the sphere at the same time even though the stars they see will be different.


[SLIDE 3]

The concept of an ever-changing center is the basis for the horizontal celestial coordinate system. In this coordinate system, we use the Earth's multiple planes -- for example, its axis, equator, and orbit -- to provide location-specific coordinates. Importantly, it is the projection of these planes out to the celestial sphere and their intersections that make up the horizontal celestial coordinate system.


[SLIDE 4]

For example, Earth's projected axis, represented by the dotted off-vertical line in the diagram, intersects with the celestial sphere at two points referred to as the celestial north and south poles. The intersection between Earth's projected equator and the celestial sphere, colored in light blue in the diagram, is known as the celestial equator. 


[SLIDE 5]

As we mentioned previously, the center of the celestial sphere is the Earth, or more precisely it is a moveable target on Earth depending on the position/view of the observer. The horizontal celestial coordinate system is, therefore, also based on moveable coordinates.

Imagine, you go outside at night to look up at the stars. A line comes up through you and up to the celestial sphere. The point at which this line intersects the celestial sphere is called the zenith. If we extend that line down through you and the Earth, it reaches the celestial sphere on the other side at the nadir. This line through you (the observer) and the Earth is the zenith-nadir axis.

Now, imagine you are looking out across a hill top and all around you are stars. Your view point becomes the astronomical horizon. It is the horizon between Earth and the sky. If you slowly turned in place in a 360 degree motion, at every step this would be your new astronomical horizon. As the zenith-nadir line is the axis going through you, and the astronomical horizon is your view out to the celestial sphere, we can describe this as a circle on the celestial sphere that is perpendicular to the zenith-nadir line.


[SLIDE 6]

So far, we have our point of reference (the zenith) and our view point (the astronomical horizon), but this still gives us a large area to navigate over. That is why we also have a celestial meridian which is another circle that intersects the zenith and the celestial poles. The celestial meridian separates the sky into two parts, East and West.


[SLIDE 7]

However, if our observer in Iceland wants to discuss with an observer in New Zealand something interesting they have found, they cannot use the horizontal celestial coordinate system as this is specific to their location, time, and date. Instead, they may use the equatorial coordinate system.

In this coordinate system, the reference points are defined by an origin at the center of the Earth, relying again on the celestial equator and on the vernal equinox. Importantly, the equatorial coordinate system is aligned to Earth's equator and poles but does not rotate with Earth; in other words, it is fixed. The equatorial coordinate system identifies a star's location using a pair of coordinates: right ascension and declination.


[SLIDE 8]

Right ascension is a measure of the angular distance to an object, passing along the celestial equator, from the vernal equinox, always travelling eastward to the hour circle passing through the object. Analogous to terrestrial longitude, right ascension is usually measured in hours, minutes and seconds instead of degrees. There are 360 degrees to every 24 hours. This equates to 15 degrees of longitude corresponding to one hour of right ascension, with a total of 24 hours of right ascension around the entire celestial equator.


[SLIDE 9]

Declination measures the angular distance of an object moving perpendicular to the celestial equator with positive to the north and negative to the south. It is analogous to geographical latitude. For example, the north celestial pole has a declination of +90 degrees while the south celestial pole has a declination of -90 degrees. Declination is measured in degrees, but can be further broken down into arcminutes and arcseconds. One degree is 60 arcminutes, and one arcminute is 60 arcseconds.


[SLIDE 10]

Now that we know how to map and reference the objects of the night sky, we will take a look at how to use a sky chart.  Sky charts are not a modern invention. The example above is a sky chart from the 17th century.


[SLIDE 11]

In contrast, the sky chart above was the actual chart used by Command Module Pilot Michael Collins during the Apollo 11 mission.


[SLIDE 12]

The accompanying exercises for this lesson are based around an online sky chart called Heavens Above. A sky chart is a map of the astronomical horizon at a given time and date. If you have a Sky Chart, no or low artificial lights, and uninterrupted 360 degree views, you can identify stars by observing the night sky. As time and date are fixed, we can also identify moving planets.


[SLIDE 13]

By default, Heaven's Above shows the sky as you would see it if you stood close to the North Pole and looked up. We can change location in the top right-hand box on the Heaven's Above page.


[SLIDE 14]

We can also change the settings to bring up a Sky Chart for a particular time and date. The center of the chart will always be the zenith, so if you take the chart outside and face south, the sun will have set or be setting to your right. With North at the top of the page, the bottom part of the chart will represent what you should be seeing. If you move around to face North, the top part of the chart will represent the view you now see. You will have a chance to try this out in the exercises.