Published on November 15, 2007
The Night Sky: The Night Sky Discussion: Discussion What are some aspects of the night sky that have caught you attention? Number of visible stars: Number of visible stars The are only about 6000 stars visible without a telescope. Stellar magnitudes: Stellar magnitudes We specify the apparent brightness of stars by the magnitude scale. Brighter stars have a smaller magnitude while fainter stars have a larger magnitude. The brightest star, Sirius has a magnitude –1.4 The faintest stars visible to the unaided eye have a magnitude of 6. Discussion: Discussion The magnitude system that astronomers use is based on the way our eyes scale the intensity of light, which is not linear but logarithmic. What do you think would be the advantage of having eyes which scale intensity in such a fashion? Diurnal (daily) motion of the stars: Diurnal (daily) motion of the stars Like the Sun, the stars generally rise in the east and set in the west. This daily motion of the stars is caused by the rotation of the Earth. Slide8: All the stars appear to circle a point in the sky called the celestial pole. In the north this point lies near the star Polaris, the north star. Circumpolar stars: Circumpolar stars Not all stars rise and set. Some stars, called circumpolar, always appear above the horizon. The celestial sphere : The celestial sphere We can picture the sky as the inside of a hemisphere. Discussion : Discussion How do we measure positions on the Earth’s surface? Positions in the sky: Positions in the sky Any position in the sky can be specified by two angles, just like latitude and longitude on the Earth’s surface. Some terminology: Some terminology Zenith – the point in the sky directly over head. Meridian – an arc drawn from due north on the horizon to the point due south on the horizon which passes through the zenith. The celestial poles and equator: The celestial poles and equator The point in the sky directly above the Earth’s north pole is called the north celestial pole. Likewise, the point directly above the south pole is called the south celestial pole. Midway between the celestial poles (90 degrees away) is the celestial equator, which lies directly above the Earth’s equator. Discussion: Discussion This there any place on Earth where all the stars are circumpolar, never falling below the horizon? Explain. Discussion: Discussion Is there any place on Earth where none of the stars are circumpolar? Explain. Discussion: Discussion If we were at the North pole, how far above the horizon would the North celestial pole be? Discussion: Discussion How high above the north horizon is the north celestial pole from Earth’s equator? Discussion: Discussion How high above the northern horizon would the north celestial pole appear in Cleveland which has a latitude of about 42 degrees? Measuring latitude: No matter where on Earth you are, you can easily determine your latitude by measuring the angle of the celestial pole from the horizon. Measuring latitude Discussion: Discussion The biggest problem in navigating using the stars was in determining one’s longitude. Explain why this is so difficult? Positions in the sky: Positions in the sky Any position in the sky can be specified by two angles. At the telescope we use the angle from the celestial equator called declination and the angle from the meridian called the hour angle. Example: Example The celestial equator has a declination of 0 degrees, while the north celestial pole has a declination of 90 degrees. The hour angle is measured in hours, 360 degrees = 24 hours. Discussion: Discussion To view an object at the zenith, to what declination and hour angle would I need to set the telescope on the roof? Remember we are at a latitude of about 42 degrees. Celestial Coordinates: Celestial Coordinates Declination (Dec) – measured in degrees minutes and seconds, from the celestial equator. Right Ascension (RA) – measured in hour minutes and seconds from the vernal equinox. Annual motion of the stars: Annual motion of the stars The same stars are not visible all year long. Any given non-circumpolar star will set 4 minutes early each day until it becomes lost in the glare of the setting Sun. This motion is caused by Earth’s yearly motion around the Sun. In other words: In other words While the Sun takes 24 hours to traverse the sky, the stars take only 23 hours and 56 minutes. The Sun moves 4 minutes eastward each day relative to the stars. Thus different stars are visible at different times of the year. The ecliptic: The ecliptic The ecliptic is the annual path through the sky that the Sun appears to take. In other words, the ecliptic is the plane of Earth’s orbit projected onto the stars.