Motion in the Sky

Discussion How do we estimate how far away things are from us in everyday life?

Parallax

Discussion Does this work with astronomical objects, like the stars, the Sun, the Moon and the planets? Can you tell by looking at them which is further from you than the others?

The celestial poles and equator north celestial pole – point in the sky directly above the Earth’s north pole south celestial pole – point in sky directly above the Earth’s south pole. celestial equator – midway between the celestial poles (90 degrees away), lies directly above the Earth’s equator.

Diurnal (daily) motion of the Sun The Sun rises in the east and sets in the west. Thus, each day it crosses the meridian. AM – ante meridian PM – post meridian When the Sun crosses the meridian it is also the farthest from the horizon.

Discussion Can you tell by observing the Moon that it is in fact closer to you than the stars? If so, how?

Occultation of Regulus

Occultation of Saturn

Discussion Why does the Moon look much bigger near the horizon than when it is high in the sky? Does it get further away as it rises?

Circumpolar stars Not all stars rise and set below the horizon. Some stars, called circumpolar, always appear above the horizon.

Southern Circumpolar stars

Fake star trail picture

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. In the south this point lies near the Southern Cross.

Star trails on the celestial equator

Discussion On Earth’s surface, also a sphere, we specify positions using latitude and longitude. How does this system work?

Discussion If we were at the North pole, how far above the horizon (in degrees) would the North celestial pole be?

Discussion How high above the north horizon in degrees is the north celestial pole from Earth’s equator?

Discussion How high above the northern horizon would the north celestial pole appear in Cleveland which has a latitude of about 42 degrees?

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

Positions in the sky If we imagine the sky as the inside surface of a sphere, we can specify any point on this celestial sphere using two coordinates.

Celestial coordinates The angle from the celestial equator is called the declination and the angle from the celestial “prime meridian” is called the right ascension. The celestial equator has a declination of 0 degrees, while the north celestial pole has a declination of 90 degrees. The RA is measured in hours where 360 degrees = 24 hours.

Earth’s rotation The diurnal motion is due to Earth’s rotation The stars take 23 hours 56 minutes from one meridian crossing to the next, this is Earth’s actual rotation period called the sidereal day

The annual motion of the Sun The Sun on the other hand, takes an average of 24 hours between successive meridian crossings. The difference is due to Earth’s revolution about the Sun. The Sun moves on average 4 minutes eastward each day relative to the stars, staying in the sky longer each day than a star at the same declination.

Solar and sidereal days Mean solar day – 24 hours Sidereal day – 23 hours 56 minutes is the actual rotation period of the Earth

Earth’s orbit speed is not constant The Earth moves faster when it is closer to the Sun (in January) and slower when it is further away (in July). This means that the length of the day is not a constant 24 hours during the year. Instead of varying the length of the day throughout the year we use the mean or average position of the Sun to define our 24-hour day.

Discussion If we stopped the Earth in its orbit, how would this change the length of the sidereal day? How would this change the length of the solar day?

Discussion If the Earth moved faster in its orbit would the length of the solar day be longer or shorter? Why?

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 relative to the Sun, until it becomes lost in the glare of the setting Sun.

The ecliptic The ecliptic is the annual path through the sky that the Sun appears to take. In actuality, the ecliptic is the plane of Earth’s orbit projected onto the stars.

The constellations In modern astronomy, the constellations are 88 irregular areas that completely cover the sky. Thus, every celestial object lies within the boundaries of a constellation.

Constellations of the Zodiac The ecliptic passes through 12 constellations (actually 13) during the year. This are know as the zodiacal constellations.

Summary Diurnal motion due to rotation of the Earth Sidereal rate is actual rotation period Position of celestial poles determined by latitude Sun, Moon and planets all move eastward relative to the stars