Solar Systems S1

post meridiem

After noon, the apparent solar time is post meridieum (after the middle of the day). If the sun crossed the meridian 3 hrs ago the apparent solar time is 3 pm.

daylight saving time

Between the second Sunday in March and the first Sunday in November, most of the US changes to daylight saving time, which is 1 hr ahead of standard time.

Why is the solar day about 4 minutes longer than the sidereal day?

Earth travels about 1 degrees per day around its orbit, a solar day requires about 1 degrees of extra rotation compared to a sidereal day. This extra 1 degree rotation takes about 1/360 of Earth's rotation period, which is about 4 minutes.

Celestial navigation in practice:

First, finding either latitude or longitude requires a tool for measuring angles in the sky. One such device, called as astrolabe was invented by the ancient Greeks and significantly improved by Islamic scholars. The astrolabe's faceplate could be used to tell time, because it consisted of rotating star map and horizon plates for specific latitudes.

universal time

For purposes of navigation and astronomy, is useful to have a single time for the entire EArth. For historical reasons, this world time was chosen to be the mean solar time in Greenwich, England--the place that also defined longitude 0. Also called universal coordinated time.

mean solar time

If we average the differences between the time your clock would read and the time a sundial would read, we can define mean solar time. Mean solar time is actually more convenient than apparent solar time. Like apparent solar time, mean solar time is a local measure of time (it varies with the longitude because of Earth's west to east location)

apparent solar time

If we base time on the Sun's actual position in the local sky, as is the case when we use a sundial, we are measuring apparent solar time. Noon is the precise moment when the Sun is highest in the sky and the sundial casts its shortest shadow.

how can you determine latitude?

It's easy to determine your latitude if you can find the north or south celestial pole in your sky, because it is equal to the altitude of the celestial pole. If you want to be more precise, you can determine your latitude from the altitude of any star as it crosses your meridian.

Jacob's staff

Medieval sailors often measured angles with a simple pair of calibrated perpendicular sticks, called a cross staff or Jacob's staff. A more modern device called a sextant allows much more precise angle determinations by incorporating a small telescope for sighting.

synodic month

One month comes from the moon's 29.5 day cycle of phases. This 29.5 day period is called a synodic month. A synodic month gets its name from the idea that the Sun and the Moon "meet" in the sky with every new moon. A synodic month is not the Moon's true orbital period.

solar day

Our 24 hour day is based on the time is takes for the Sun to make one circuit around the local sky. You can measure this time period by measuring how long it takes the Sun to go from the highest point in the sky one day to its highest point the next day.

tropical year

Our calendar is based on the cycle is based on the cycle of the seasons, which we measure as the time from the spring equinox one year to the spring equinox the next year. This time period, called a tropical year, is about 20 minutes shorter than the sidereal year.

When and why do we have leap years?

Our modern calendar is designed to stay synchronized with the seasons and is therefore based on the tropical year. Because the length of a year is about 365.25 days (rather than exactly 365 days), the egyptian calendar drifted out of phase with the seasons by about 1 day every 4 years. To keep the seasons and the calendar synchronized, Caesar decreed the adoption of a new calendar in 46 BC.

sidereal month

Represents the moon's true orbital period, about 27.333 days. Like the sidereal day, the sidereal month gets its name from the fact that it describes how long it takes the Moon to complete an orbit relative to the positions of distant stars.

Julian calendar

The Julian calendar introduced the leap year: every fourth year has 366 days, rather than 365, so that the average length of the calendar is 365.25

celestial equator

The celestial equator always extends from due east on your horizon to due west on your horizon, crossing the meridian at an altitude of 90 degrees minus your latitude

What's the difference between a sidereal and a tropical year?

The difference between the sidereal and the tropical year arises from Earth's 26,000 year cycle of axis precession. Precession not only changes the orientation of the axis in space but also changes the location in Earth's orbit at which the seasons occur. Each year, the location of the equinoxes and soltices shift 1/26,000 of year.

standard time

The growth of railroad travel made mean solar time increasingly problematic. In 1883, the railroad companies agreed to a new system that divided the US into four time zones, setting all clocks within each zone to the the same time. That was the birth of standard time, which today divides the world into time zones. Time zones often have unusual shapes to conform to social, economic, and political realities, so larger variations between standard time and mean solar time sometimes occur.

the Arctic Circle

The special latitudes at which the Sun remains continuously above the horizon for a full day each year are the polar circles--arctic circle at 66.5 N

the global positioning system

Today, a new type of celestial navigation has supplanted traditional methods. It finds positions relative to satellites of the global positioning system.

transits

WE see Mercury or Venus appear to pass directly across the face of the Sun during inferior conjunction. Mercury transits occur an average of 12 times per century. Venus transits comes in pairs 8 years apart.

sidereal year

We can also define a year in two slightly different ways. The time it takes Earth to complete one orbit relative to the stars is called a sidereal year.

how can you determine your longitude?

You can determine your longitude by comparing the current position of an object in your sky with its position as seen from some known longitude.

sidereal day

You can measure Earth's rotation period by measuring how long it takes for any star to go from its highest point in the sky one day to its highest point the next day. The sidereal day is about 23 hrs and 56 minutes. Sidereal means "related to the to the stars."

Anarctic Circle

at the latitude at 66. 5 S

ante meridiem

before noon, when the sun is rising upward through the sky, the apparent solar time is ante meridian (before the middle of the day. If the sun will reach the meridian 2 hrs from now, the apparent solar time is 10 a.m.

tropic of Cancer

circle of latitude--23.5 degrees N, marks the northernmost latitude at which the Sun ever reacher the zenith.

tropic of Capricorn

circle of latitude--23.5 degrees S, the sun reaches the zenith at local noon on the winter solstice, making this the southernmost latitude at which the Sun ever reaches the zenith

How do stars move in the local sky?

depends on (1) your latitude (2) the declination of the star

spring equinox

dual meaning--moment when spring begins and the point on the ecliptic at which the Sun appears to be located at that moment

greatest elongation

important for inner planet--two other points when they appear farthest from the sun in our sky

stars w/ a dec=0

lie on the celestial equator and therefore rise due east, cross the meridian at the zenith, and set due west

Gregorian calendar

much like the Julian calendar, with two important adjustments. First, Pope Gregory decreed that the day in 1582 following October 4 would be October 14. Second, the Gregorian calendar added an exception to the rule of having leap year every 4 years. Today, the Gregorian calendar is used worldwide for international communication and commerce.

declination

our system of celestial coordinates--similar to latitude on Earth. Just as the lines of latitude are parallel to Earth's equator, lines of declination are parallel to the celestial equator. Just as Earth's equator has a lat = 0, the celestial equator has dec = 0. Latitude is labeled north to south relative to the equator, while declination is labeled positive or negative. The fraction of any star's daily circle that is above the horizon--and hence the amount of time it is above the horizon each day--depends on its declination.

right ascension

our system of celestial coordinates--similar to longitude. Lines of right ascension extend from the north celestial pole to the south celestial pole. Just as there is no natural starting point for longitude, there is no natural starting point for right ascension. Right ascension zero is the line of right ascension that runs through the spring equinox. Longitude is measured in degrees east or west of Greenwich, while right ascension is measured in hour east of the spring equinox. Generalizing, an objects right ascension tells us how long after the spring equinox the object crosses the meridian. Right ascension affect only the time of day and year at which a star is found in a particular position in your sky.

opposition

planet appears exactly opposite the sun in our sky--the planet moves through the sky like the full moon, rising at sunset, reaching the meridian at midnight, and setting at dawn. The planet is closest to Earth at opposition and hence appears brightest in our sky at this time. (for a more distant planet)

conjunction

planet lines up with the Sun--we cannot see the planet because it is hidden by the Sun's glare and rises and sets with the Sun in our sky. (for a more distant planet)

inferior and superior conjunction

refers to a planet that is nearer than Earth to Sun

stars w/ dec >0

rise north of due east, reach their highest point on the meridian in the north, and set north of due west. Their rise, set, and highest point depend on their declination.

Stars w/ dec <0

rise south of due east, reach their highest point on the meridian in the south, and set south of due west.

features of the sky for a latitude 40 degrees

stars w/ dec=0 lie on the celestial equator and therefore follow the celestial equator's path through the local sky. stars w/ greater than 90 are circumpolar. Stars w/ greater than 0 that are not circumpolar. Stars w/ dec less than -90 never rise above the horizon. Stars the dec less than 0 that are sometimes visible follow paths parallel to but south of the celestial equator.

tropics

the region between two circles--the tropic of cancer and capricorn is called the tropics and represents the parts of EArth where the Sun can sometimes can sometimes reach the zenith at noon

synodic period

the time from when it is lined up with the Sun in our sky once to the next familiar alignment

sidereal period

the time the planet takes to orbit the Sun--measured relative to teh stars

planisphere

today you can buy similar rotatable maps

celestial coordinates

we can make the celestial sphere even more useful by giving it a set of celestial coordinates that function much like the coordinates of latitude and longitude on Earth