S1 Flashcards
How do we define the day, month, year, and planetary periods?
Each of these is defined in two ways. A sidereal day is Earth’s rotation period, which is about 4 minutes shorter than the 24-hour solar day from noon one day to noon the next day. A sidereal month is the Moon’s orbital period of about days; a synodic month is the days required for the Moon’s cycle of phases. Asidereal year is Earth’s orbital period, which is about 20 minutes longer than the tropical year from one March equinox to the next. A planet’s sidereal period is its orbital period, and its synodic period is the time from one opposition or conjunction to the next.
How do we tell the time of day?
There are several time measurement systems. Apparent solar time is based on the Sun’s position in the local sky. Mean solar time is also local, but it averages the changes in the Sun’s rate of motion over the year. Standard time and daylight saving time divide the world into time zones. Universal time is the mean solar time in Greenwich, England.
How do we locate objects on the celestial sphere?
Declination is given as an angle describing an object’s position north or south of the celestial equator. Rightascension, usually measured in hours (and minutes and seconds), tells us how far east an object is located relative to the March equinox.
How do stars move through the local sky?
A star’s path through the local sky depends on its declination and your latitude. Latitude tells you the orientation of your sky relative to the celestial sphere, while declination tells you how a particular star’s path compares to the path of the celestial equator through your sky.
How does the Sun move through the local sky?
The Sun’s path also depends on its declination and your latitude, but it varies throughout the year because of the Sun’s changing declination. TheSun’s varying path helps define special latitudes, including the tropic of Cancer and tropic of Capricorn and the Arctic Circle and Antarctic Circle.
How can you determine your latitude?
You can determine your latitude from the altitude of the celestial pole in your sky or by measuring the altitude and knowing the declination of a star (or the Sun) as it crosses your meridian.
How can you determine your longitude?
To determine longitude you must know the position of the Sun or a star in your sky and its position at the same time in the sky of Greenwich, England (or some other specific location). This is most easily done if you have a clock that tells universal time.
