Guiding Questions What role did astronomy play in ancient civilizations? Are the stars that make up a constellation actually close to one other? Are the same stars visible every night of the year? What is so special about the North Star? Are the same stars visible from any location on Earth? What causes the seasons? Why are they opposite in the northern and southern hemispheres? Has the same star always been the North Star? Can we use the rising and setting of the Sun as the basis of our system of keeping time? Why are there leap years?

Naked-eye astronomy had an important place in ancient civilizations. Chichén Itzá in Yucatán Stonehenge in British Isles Medicine Wheel in Wyoming Casa Grande in Arizona

Eighty-eight constellations cover the entire sky.

Eighty-eight constellations cover the entire sky.

Eighty-eight constellations cover the entire sky. 6000 stars visible to unaided eye (only half are above the horizon). 88 semi-rectangular groups of stars called constellations. Some stars in the constellations are quite close while others are very far away.

Eighty-eight constellations cover the entire sky Eighty-eight constellations cover the entire sky. Constellation names are derived from the myths and legends of antiquity.

The appearance of the sky changes during the course of the night and from one night to the next.

Diurnal Motion of the Night Sky Each night, most stars appear to rise in the east, move across the sky, and set in the west because of Earth’s rotation.

Looking toward the North The North Star (Polaris) does not appear to move. Stars in the northern sky seem to move in a counter-clockwise sense. Northern stars that never set are called circumpolar stars.

As Earth orbits our Sun, different constellations are visible at different times of the year. The circumpolar constellations are always the same because they are visible no matter where Earth is in its orbit.

It is convenient to imagine that the stars are located on a celestial sphere. Celestial equator: splits the sky into a northern half and a southern half. North celestial pole: the point directly above Earth’s rotation axis.

Celestial Coordinates pinpoint positions on the celestial sphere. Right Ascension: how far objects are to the east of the vernal equinox (Sun’s position on March 21). Declination: how far objects are above or below the celestial equator.

The point directly overhead is called the ZENITH The point directly overhead is called the ZENITH. The line that splits the sky into eastern and western halves is called the MERIDIAN.

The celestial sphere seems to spin around the Earth. The horizon is the line that separates what can be seen in the sky and what cannot. For observers on Earth at a latitude of 35º, the NCP is located at an altitude of 35º above the horizon. Stars near the SCP are never visible.

Over the course of a year, the Sun’s position in the sky changes.

The Sun’s daily path across the sky

Dec. 21 Winter Solstice March 21 Vernal Equinox June 21 Summer Solstice Sept. 21 Autumnal equinox

The ecliptic is the Sun’s apparent path around the celestial sphere over the course of a year.

The ecliptic is the Sun’s apparent path around the celestial sphere over the course of a year.

The seasons are caused by the tilt of Earth’s axis of rotation.

The seasons are caused by the tilt of Earth’s axis of rotation.

The Moon helps to cause precession, a slow, conical motion of Earth’s axis of rotation.

12,000 years from now, the bright star Vega will be the “new North Star” because of precession.

Positional astronomy plays an important role in keeping track of time. Key Question: When is the Sun on the meridian (directly in the south)? Apparent solar day: the interval between two successive meridian transits of the Sun (varies around 24 hrs as Earth orbits the Sun at varying speeds). Mean solar day: the interval between two successive meridian transits of the Sun IF it moved at a constant rate (exactly 24 hrs). Sidereal time: the interval of time between two successive meridian transits of a star (23 hrs 56 min).

When is the Sun overhead at my location?

Astronomical observations led to the development of the modern calendar Our Sun takes 365.24220 days to move around the celestial sphere once (one year). 0.24220 fractional days is 5 hours, 48 minutes, and 46 seconds – a fraction that has caused endless headaches for calendar makers who would rather the year was exactly 365 days long! In 45BC Julius Caesar decreed that years are 365 days with one extra day added in February, every four years (good to one day in 128 years). In 1582, Pope Gregory XIII introduced the currently used Gregorian calendar that does not allow leap years in Centuries unless the year is evenly divisible by 400 (good to one day in 3300 years).

Guiding Questions What role did astronomy play in ancient civilizations? Are the stars that make up a constellation actually close to one other? Are the same stars visible every night of the year? What is so special about the North Star? Are the same stars visible from any location on Earth? What causes the seasons? Why are they opposite in the northern and southern hemispheres? Has the same star always been the North Star? Can we use the rising and setting of the Sun as the basis of our system of keeping time? Why are there leap years?