The Night Sky

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.

North celestial pole

South celestial pole

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.

Circumpolar stars

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

Star trails on the celestial equator

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.

Discussion

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

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

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

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.

Solar and sidereal days

Solar day – 24 hours

Sidereal day – 23 hours 56 minutes is the actual rotation period of the Earth

Discussion

Which star (A, B, C, or D) will spend the least amount of time above the horizon during the course of a day?

What is the latitude of this picture?

How long do stars on the celestial equator spend above the horizon?

If taken in the northern hemisphere, are the stars rising or setting in this picture?

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.

Discussion

About what time is it in this picture?

Discussion

In what constellation will the Sun be in at 6 pm?

Discussion

In what constellation will the Sun be in one month from the time of this picture?

The Seasons

Due to the Earth’s rotation axis being tilted by 23.5 degrees from perpendicular to the plane of its orbit.

Conservation of angular momentum

Anything that spins on an axis or revolves around another object has angular momentum.

Conservation of angular momentum requires that the rate of spinning remains constant with time. Also, the axis of rotation of any spinning object remains in a fixed direction in space.

Near edge-on view

The seasons and ellipticity

The Earth’s orbit is nearly circular – distance from the Sun varies by only 3%

Earth is closest to the Sun in January and furthest from the Sun in July

Summer in northern hemisphere is winter in southern hemisphere

The arctic circle

The arctic circle is the northern latitude at which on the summer solstice the Sun never sets and on the winter solstice the Sun never rises above the horizon.

Discussion

What is the latitude of the arctic circle?

Discussion

The tropics mark the farthest points north and south where the Sun can appear at the zenith. What is the latitude of the tropics?

Why is summer warmer?

1. The Sun, being above the celestial equator, remains in the sky longer during the summer, the longest daylight time occurring on the summer solstice.

2. The Sun rays hit the earth more directly during the summer months, i.e. the summer hemisphere receives more energy per square meter.

Which planet has most the extreme seasons?

Discussion

If the Earth’s rotation axis were exactly perpendicular to the ecliptic, would we still experience seasons?

Precession: a complication

The Earth’s rotation axis is not fixed in space over long periods of time

The Earth’s rotation rate is nearly constant, its speed is not

The speed at which points on the Earth’s equator are moving is larger than points on the Earth at higher latitudes. At the equator you would be moving at 1,650 km/hr, while at the north pole you would not be moving at all, just rotating around a point.

The extra velocity at the equator pushes the matter out at the equator.

The same thing happens on a merry-go-round. The closer you are to the edge, the faster you will be moving, and the greater the force pushing you off.

Equatorial Bulge

The Earth is not a perfect sphere

The Earth bulges out at its equator, that is, its diameter measured along its equator is 43 km larger than its diameter from the north to the south pole.

Oblateness

Precession

Because the Earth is not a perfect sphere and its rotation axis is tilted 23.5 degrees from its orbital plane, the Sun and the Moon pull on the extra mass in Earth’s equatorial bulge and try to straighten out the tilt.

The Sun and Moon apply a torque to the Earth

Precession

This off axis force, or torque on the Earth causes Earth’s rotation axis to vary slightly, or precess, over long periods of time.

Thus, the celestial poles trace out a circle against the stars over a period 26,000 years.

Precession of the Equinoxes

If the celestial poles change with time, so to must the celestial equator, which is 90 degrees away from the poles. If the celestial equator changes with time, than the intersection of ecliptic and the celestial equator will also change with time.

Age of Aquarius?

The “age” is defined by the constellation the Sun is in on the vernal equinox. Currently, it is the age of Pisces.

Discussion

Why is the changing of the equinox position important?

Tropical year

The length of our calendar year is defined by the time it takes between two vernal equinoxes. This is called the tropical year and is 365 days 5 hours 48 minutes and 46 seconds long.

Sidereal year

However, because the equinox positions slowly changes, this is not the Earth’s actual orbital period around the Sun. The actual orbital period is called the sidereal year and is 365 days, 6 hours, 9 minutes and 10 seconds, or 20 minutes 24 seconds longer.

Leap Years

Notice that our tropical year is not an even number of days. The extra 5 hours, 48 minutes and 46 seconds is almost, but not quite ¼ of a day. To make up for this, every 4 years we have to add an extra day to our calendar.

What is the difference?

If we used the sidereal year for our calendar year the seasons would slowly migrate through the different months over time. Spring would come one day earlier every 70 years.

Astronomical coordinates

Because of precession and the changing position of the celestial poles and the vernal equinox, celestial coordinates of astronomical objects change from year to year.

Astrology

Precession breaks the cause and effect relationship between the position of the stars and seasonal changes.

TRADITIONALASTROLOGICAL DATES

ACTUAL ASTRONOMICAL DATESTODAY (c. 2000 C.E.)

Constellation Date Span Days Constellation Date Span Days

Aries 21 Mar - 19 Apr 30 Aries 19 Apr - 13 May 25

Taurus 20 Apr - 20 May 31 Taurus 14 May - 19 Jun 37

Gemini 21 May - 20 Jun 31 Gemini 20 Jun - 20 Jul 31

Cancer 21 Jun - 22 Jul 32 Cancer 21 Jul - 9 Aug 20

Leo 23 Jul - 22 Aug 31 Leo 10 Aug - 15 Sep 37

Virgo 23 Aug - 22 Sep 31 Virgo 16 Sep - 30 Oct 45

Libra 23 Sep - 22 Oct 30 Libra 31 Oct - 22 Nov 23

Scorpio 23 Oct - 21 Nov 30 Scorpius 23 Nov - 29 Nov 7

Ophiuchus 30 Nov - 17 Dec 18

Sagittarius 22 Nov - 21 Dec 30 Sagittarius 18 Dec - 18 Jan 32

Capricorn 22 Dec - 19 Jan 29 Capricornus 19 Jan - 15 Feb 28

Aquarius 20 Jan - 18 Feb 30 Aquarius 16 Feb - 3/11 24-25

Pisces 19 Feb - 20 Mar 30-31 Pisces 12 Mar - 18 Apr 38