The Celestial Sphere The 88 official constellations cover the celestial sphere. If you do not have a model of the celestial sphere to bring to class, you.

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The Celestial Sphere The 88 official constellations cover the celestial sphere. If you do not have a model of the celestial sphere to bring to class, you might wish to use this slide; you will probably want to skip it if you have a model that you can discuss instead…

Constellations A constellation is a region of the sky.
88 constellations fill the entire sky. Remind students that we often use the term “constellation” to describe a pattern of stars, such as the Big Dipper or the stars that outline Orion. However, technically a constellation is a region of the sky (and the patterns are sometimes called “asterisms”). A useful analogy for students: a constellation is to the sky as a state is to the United States. That is, wherever you point on a map of the U.S. you are in some state, and wherever you point into the sky you are in some constellation.

The Local Sky An object’s altitude (above horizon) and direction (along horizon) specifies its location in your local sky Now we move from the celestial sphere to the local sky. The local sky looks like a dome because we see only half the celestial sphere. If we want to locate an object: It’s useful to have some reference points. Students will probably already understand the horizon and the cardinal directions, but explain the zenith and the meridian; a simple way to define the meridian is as an imaginary half-circle stretching from the horizon due south, through the zenith, to the horizon due north. Now we can locate any object by specifying its altitude above the horizon and direction along the horizon. A good way to reinforce this idea is to pick an object located in your class room, tell students which way is north, and have them estimate its altitude and direction.

The Local Sky Zenith: The point directly overhead
Horizon: All points 90° away from zenith Meridian: Line passing through zenith and connecting N and S points on horizon Now we move from the celestial sphere to the local sky. The local sky looks like a dome because we see only half the celestial sphere. If we want to locate an object: It’s useful to have some reference points. Students will probably already understand the horizon and the cardinal directions, but explain the zenith and the meridian; a simple way to define the meridian is as an imaginary half-circle stretching from the horizon due south, through the zenith, to the horizon due north. Now we can locate any object by specifying its altitude above the horizon and direction along the horizon. A good way to reinforce this idea is to pick an object located in your class room, tell students which way is north, and have them estimate its altitude and direction.

Celestial Coordinates
Right ascension: Like longitude on celestial sphere (measured in hours with respect to spring equinox). Declination: Like latitude on celestial sphere (measured in degrees above celestial equator)

Celestial Coordinates of Vega
Right ascension: Vega’s RA of 18h35.2m (out of 24h) places most of the way around celestial sphere from spring equinox. Declination: Vega’s dec of +38°44’ puts it almost 39° north of celestial equator (negative dec would be south of equator)

Celestial Coordinates of Sun
The Sun’s RA and dec change along the ecliptic during the course of a year Sun’s declination is negative in fall and winter and positive in spring and summer

How do we define the day, month, year, and planetary time periods?

Length of a Day Sidereal day: Earth rotates once on its axis in 23 hrs, 56 min, and 4.07 sec.

Length of a Day Solar day: The Sun makes one circuit around the sky in 24 hours

Length of a Month Sidereal month: Moon orbits Earth in 27.3 days.
Earth & Moon travel 30° around Sun during that time (30°/360° = 1/12) Synodic month: A cycle of lunar phases; therefore takes about 29.5 days, 1/12 longer than a sidereal month

The sky varies as Earth orbits the Sun
As the Earth orbits the Sun, the Sun appears to move eastward along the ecliptic. At midnight, the stars on our meridian are opposite the Sun in the sky. Use this interactive figure to explain how the constellations change with the time of year.

Parallax and Distance

The Magnitude Scale

Length of a Year Sidereal year: Time for Earth to complete one orbit of Sun Tropical year: Time for Earth to complete one cycle of seasons Tropical year is about 20 minutes (1/26,000) shorter than a sidereal year because of Earth’s precession.

Mean Solar Time Length of an apparent solar day changes during the year because Earth’s orbit is slightly elliptical. Mean solar time is based on the average length of a day. Noon is average time at which Sun crosses meridian It is a local definition of time

The Analemma The analemma illustrates position of Sun with respect to mean solar time

Universal Time Universal time (UT) is defined to be the mean solar time at 0° longitude. It is also known as Greenwich Mean Time (GMT) because 0° longitude is defined to pass through Greenwich, England It is the standard time used for astronomy and navigation around the world

Standard Time & Time Zones
Rapid train travel required time to be standardized into time zones (time no longer local)

Local sidereal time = RA + hour angle
Time by the Stars Sidereal time is equal to right ascension that is passing through the meridian Thus, the local siderial time is 0h0m when the spring equinox passes through the meridian A star’s hour angle is the time since it last passed through the meridian Local sidereal time = RA + hour angle

How do stars move through the local sky?
Coming Next Week…! (and Stellarium Demos) How do stars move through the local sky? A star’s path depends on your latitude and the star’s declination

Star Paths in Northern Hemisphere
In north, stars with dec > 90° - (your latitude) are circumpolar Celestial equator is in south part of sky

How does the Sun move through the local sky?
Sun’s path is like that of a star, except that its declination changes over the course of a year

Altitude of the celestial pole = your latitude
Show students how to locate the NCP and SCP, and how the sky moves around them. (You might wish to repeat the time exposure photo of the sky at this point to re-emphasize what we see.) Can also ask students where they’d find the north celestial pole in their sky tonight…

How can you determine your latitude?
Latitude equals altitude of celestial pole Altitude and declination of star crossing meridian also gives latitude.

Latitude During Daytime
You can determine the Sun’s declination from the day of the year Thus, measuring the Sun’s altitude when it crosses meridian can tell you latitude

How can you determine your longitude?
In order to determine your longitude from the sky, you need to know time of day because of Earth’s rotation You also need to know day of year because of Earth’s orbit Accurate measurement of longitude requires an accurate clock.

Summary: How do we locate objects on the celestial sphere?
Each point on the celestial sphere has a particular right ascension (like longitude) and declination (like latitude). How do stars move through the local sky? Their paths depend on your latitude and the star’s declination. How does the Sun move through the local sky? Sun moves like a star except its declination depends on the time of year.

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