ASTR 207 S 1.1

How does the sun move in sky?

During the day, both sun and stars rise in the east and set in the west.

The sun’s altitude will gradually change, can be seen through ecliptic and analemma

What is the altitude of the sun when it crosses the Merdian?

90 degrees

The Sun at the equator explanation

The sun passes through the zenith at apparent solar noon on the equinoxes.

Sun’s location at spiral latitudes explanation

Due to earth’s axis being tilted, there are certain latitudes with properties worth noting when researching the sun’s yearly path.

The sun at tropics

At the tropic of cancer, the sun passes through the zenith on the June solstice

At the tropic of capricorn, the sun passes through on the december solstice.

The Tropic of Cancer

the most northerly circle of latitude on Earth at which the Sun can be directly overhead

• Located approximately 23 degrees N

• June

The tropic of Capricorn

The most southernmost point on Earth where the sun can be directly overhead.

• Latitude approximately 23 Degrees S

The Sun at north and south poles

The sun circles the horizon on the equinoxes.

• It circles the skyon June/December solstices at an altitude of 23.5 degrees.

Sidereal Day

Earth rotates once on its axis in 23h, 56m, and 4.07s

Solar Day

The sun makes one circuit around the sky in 24 hours

Solar day & sidereal day comparison

A solar day is longer than a sidereal day by about 1/360th because earth moves about 1 degree in its orbit each day/

Sidereal Month

Moon orbits Earth in 27.3 days.

• Earth and moon travel 30 degrees around Sun during that time (30degrees/360degrees = 1/12)

Synodic Month

A cycle of lunar phases; takes about 29.5 days, 1/12 longer than a sidereal month.

Sidereal Year

Time for earth to complete one orbit of Sun relative to distant stars

Tropical year

Time for earth to complete one day cycle of seasons.

• A tropical year is about 20 minutes (1/26,000) shorter than a sidereal year due to precession.

Common year

365 days

Sidereal period

The amount of time it takes a planet to orbit the sun, measured with respect to the background stars.

Synodic Period

The time from when a planet is aligned with the sun in our sky until it is again aligned similarly, (such as between two conjunctions)

Transits

• A transit is seen when Mercury or Venus are at an inferior conjunction, while also being in the Earth-Sun plane.

• Mercury transits occur about 12 times a century.

  • Venus transits come in pairs 8 years apart, with more than a century between pairs.

Apparent solar time

“Noon” occurs when the sun crosses your meridian.

Mean Solar Time

Noon is set to the average time when the sun crosses your meridian over the course of the whole year.

• This accounts for subtle differences in day length.

• Based on the average length of a day.

  • Solar day changes during the year because earth’s orbit is slightly elliptical and because earth is tilted.

The Analemma

Illustrates the difference between apparent solar time and the mean solar time across the year.

Standard time (clock time)

The mean solar time in the centre of your time zone.

• Applies throughout your time zone

Universal Time

UT defined to be the mean solar time at 0 degrees longitude.

• it is known as greenwich Mean time because 0 degree longitude is defined to pass through Greenwich, England.

  • It is the standard time used for astronomy and navigation around the world.

Daylight Savings

Standard time can be bumped up forward to make better use of the sunlight, because the sun stays up longer at extreme latitudes.

• Reversed back in fall

  • Alberta will stay on permanent time.

Leap years explanations

Calendar with 365 days will get out of alignment with the seasons by roughly 1 day every 4 years.

• Leap years shall be years divisible by 4, except for years divisible by 100 but years divisible by 400, shall once again be leap years.