Motion in the Sky

Discussion What is a year?

Discussion What is a day?

Discussion Why are there 7 days in a week?

Time Day – Time it takes the Sun to return to the same position above the horizon. Month – Time it takes the Moon to go through its phases. Year – Time it takes the Sun to return it the same position relative to the stars. Note: Ancient definitions differ from the modern usage!

Discussion Do you know how the hour was originally defined? An hour is the time it takes the Moon to move its diameter against the stars.

Fixed stars and Planets Although the stars change their positions over the course of a night and from night to night, their relative positions (their positions relative to each other) remain constant year after year. Planet means “wanderer.” They change their positions relative to that of the fixed stars

The Planets of antiquity Sun Moon Mercury Venus Mars Jupiter Saturn

The days of the Week Sunday – the Sun’s day Monday – The Moon’s day Tuesday – Tiw’s day – Mars Wednesday – Woden’s day – Mercury Thursday – Thor’s day – Jupiter Friday – Freia’s day – Venus Saturday – Saturn’s day

Ordering of the day names Planets ordered by apparent length of period of revolution: Saturn Jupiter Mars Sun Venus Mercury Moon

Assign each planet to an hour of the day: Saturn 1, Jupiter 2, Mars 3, Sun 4, Venus 5, Mercury 6, Moon 7, Saturn 8, Jupiter 9, … What is the first hour of each day of the week? Discussion

How do we estimate how far away things are from us?

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 farther from you than the others?

Discussion At any given time we can only see at most half of the celestial sphere. How come?

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 do you think the Moon looks much bigger near the horizon than when it is high in the sky?

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.

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

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 Where on Earth would all visible stars be circumpolar?

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

Discussion Where on Earth would none of the visible stars be circumpolar?

Discussion How high above the north horizon 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.

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

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 the Earth moved faster in its orbit would the length of the sidereal day be longer or shorter? How about the solar day? Explain. Hint: How would the length of the days change if we were to stop Earth in its orbit?

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 traditionally through 12 constellations during the year. (In modern astronomy it is actually 13 constellations.) These 12 constellations are know as the zodiacal constellations.

Discussion Based on the previous slide, in which constellation is the Sun in today? Which constellation is high in the sky at midnight?

Earth’s axial tilt & the seasons

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