daily motion of the sun daily motion (diurnal motion) along a circle with center on the earth’s...

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Daily Motion of the Sun Daily motion (diurnal motion) along a circle with center on the Earth’s axis Clockwise in the north temperate zone Counterclockwise in the south temperate zone Up and down in the tropics Don’t worry about CW or CCW in the tropics

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Page 1: Daily Motion of the Sun Daily motion (diurnal motion) along a circle with center on the Earth’s axis Clockwise in the north temperate zone Counterclockwise

Daily Motion of the Sun

Daily motion (diurnal motion) along a circle with center on the Earth’s axis

Clockwise in the north temperate zone Counterclockwise in the south

temperate zone Up and down in the tropics Don’t worry about CW or CCW in the

tropics

Page 2: Daily Motion of the Sun Daily motion (diurnal motion) along a circle with center on the Earth’s axis Clockwise in the north temperate zone Counterclockwise

Yearly Motion of the Sun Yearly motion (annual motion) along the

ecliptic The ecliptic great circle is the

intersection of the celestial sphere with the ecliptic plane

The Sun drifts eastward (counterclockwise when viewed from above the NCP) with respect to the stars in the course of the year

Page 3: Daily Motion of the Sun Daily motion (diurnal motion) along a circle with center on the Earth’s axis Clockwise in the north temperate zone Counterclockwise

Yearly Motion of the Sun 2

Page 4: Daily Motion of the Sun Daily motion (diurnal motion) along a circle with center on the Earth’s axis Clockwise in the north temperate zone Counterclockwise

Yearly Motion of the Sun 3

We cannot see the ecliptic, but we deduce its position by considering which stars are close to the Sun at the time of sunrise or sunset

Page 5: Daily Motion of the Sun Daily motion (diurnal motion) along a circle with center on the Earth’s axis Clockwise in the north temperate zone Counterclockwise

Heliacal Rising

Heliacal (/hɪˈlaɪəkəl/) rising is the first visible appearance of a star on the eastern horizon before sunrise

On the previous morning, sunlight made the star invisible

Page 6: Daily Motion of the Sun Daily motion (diurnal motion) along a circle with center on the Earth’s axis Clockwise in the north temperate zone Counterclockwise

Dog Days

Sirius is the brightest star in the night sky

In ancient Egypt, the helical rising of Sirius coincided with the annual flooding of the Nile

Sirius is also known as the Dog Star because of its prominence in the constellation Canis Major (Big Dog)

Page 7: Daily Motion of the Sun Daily motion (diurnal motion) along a circle with center on the Earth’s axis Clockwise in the north temperate zone Counterclockwise

Dog Days 2

The Dog Days refers to the hottest days of summer

At the time of the ancient Greeks this were the days when star Sirius rose just before or at the same time as sunrise (heliacal rising)

Page 8: Daily Motion of the Sun Daily motion (diurnal motion) along a circle with center on the Earth’s axis Clockwise in the north temperate zone Counterclockwise

Equinoxes and Solstices

The equinoxes are the intersections of the celestial equator and the ecliptic

The solstices are the points on the ecliptic furthest from the celestial equator

These geocentric definitions agree with the heliocentric ones given earlier

Page 9: Daily Motion of the Sun Daily motion (diurnal motion) along a circle with center on the Earth’s axis Clockwise in the north temperate zone Counterclockwise

Equinoxes and Solstices 2

Page 10: Daily Motion of the Sun Daily motion (diurnal motion) along a circle with center on the Earth’s axis Clockwise in the north temperate zone Counterclockwise

Equinoxes and Solstices 3 The spring equinox is both a time and a

place! The spring equinox is the moment when

the Sun is at the spring equinox You finish the race when you reach the

finish! The spring equinox is the intersection

point where the counterclockwise motion is up

Page 11: Daily Motion of the Sun Daily motion (diurnal motion) along a circle with center on the Earth’s axis Clockwise in the north temperate zone Counterclockwise

Equinoxes and Solstices 4 At the time of the equinoxes, day and

night are equally long everywhere on Earth

Equinox = equal night At the time of the solstices, the rising

position of the Sun reaches its extreme northern or southern positions, so the Sun stands still before turning

Solstice = Sun standing Tropic = (Sun) turning

Page 12: Daily Motion of the Sun Daily motion (diurnal motion) along a circle with center on the Earth’s axis Clockwise in the north temperate zone Counterclockwise

Equinoxes and Solstices 5 Apparent diameter of the Sun is 0.5° At the equinox the centre of the Sun is

above the horizon for 12 hours Sunrise is defined to be when the upper

limb of the Sun crosses the horizon, so day is actually longer than night at the equinox

In addition, refraction bends light over the horizon

Page 13: Daily Motion of the Sun Daily motion (diurnal motion) along a circle with center on the Earth’s axis Clockwise in the north temperate zone Counterclockwise

Equinoxes and Solstices 6 The change in rising position of the Sun

along the horizon is fastest around the equinoxes, and slowest around the solstices

Around the equinoxes, the declination (distance from the celestial equator) will change by 0.5° per day

Near the solstices, it will stay fixed for almost a week

Page 14: Daily Motion of the Sun Daily motion (diurnal motion) along a circle with center on the Earth’s axis Clockwise in the north temperate zone Counterclockwise

Celestial Coordinates In order to set up a coordinate system

on a sphere, we need a great circle and a base point on it

On the Earth, there is only one natural great circle, on the Celestial Sphere there are three: horizon, celestial equator and ecliptic

There are therefore three coordinate systems on the celestial sphere

Page 15: Daily Motion of the Sun Daily motion (diurnal motion) along a circle with center on the Earth’s axis Clockwise in the north temperate zone Counterclockwise

Horizon Coordinate System Base circle: horizon, base point:

north point Altitude and azimuth Azimuth is measured clockwise

from 0° to 360° and tracks the daily path of the Sun in the north temperate zone

90 – altitude is called the zenith distance

Page 16: Daily Motion of the Sun Daily motion (diurnal motion) along a circle with center on the Earth’s axis Clockwise in the north temperate zone Counterclockwise

Horizon Coordinate System 2

Page 17: Daily Motion of the Sun Daily motion (diurnal motion) along a circle with center on the Earth’s axis Clockwise in the north temperate zone Counterclockwise

Equatorial Coordinate System

Base circle: celestial equator, base point: vernal equinox

Declination and right ascension RA is measured counterclockwise

in hours from 0h to 24h and tracks the time difference in meridian passage of stars

Page 18: Daily Motion of the Sun Daily motion (diurnal motion) along a circle with center on the Earth’s axis Clockwise in the north temperate zone Counterclockwise

Equatorial Coordinate System 2

Page 19: Daily Motion of the Sun Daily motion (diurnal motion) along a circle with center on the Earth’s axis Clockwise in the north temperate zone Counterclockwise

Ecliptic Coordinate System Base circle: ecliptic, base point: vernal

equinox (Celestial) latitude and (Celestial)

longitude Longitude is measured

counterclockwise from 0° to 360° and tracks the Sun’s annual motion

Celestial latitude and longitude is not relative to the celestial equator!

Page 20: Daily Motion of the Sun Daily motion (diurnal motion) along a circle with center on the Earth’s axis Clockwise in the north temperate zone Counterclockwise

Comparing Coordinate Systems Azimuth is measured in the opposite

direction from RA and longitude Horizon system is local, the two other are

global For stars, the horizon coordinates change

with time, while the two other are fixed Horizon for simple observations Equatorial for tracking stars Ecliptic for tracking Sun, Moon and

planets

Page 21: Daily Motion of the Sun Daily motion (diurnal motion) along a circle with center on the Earth’s axis Clockwise in the north temperate zone Counterclockwise

Comparing Coordinate Systems 2

The latitude of the Moon is always less than 5°

For the planets, the latitudes are less than 3° except for Mercury, which can go up to 7°

The ex-planet Pluto could go up to 17°