Lecture Series 1: Basic Facts

1. Lecture Series 1: Basic Facts • What is the universe? • Answer: All of space, time, matter, and energy. • What is Astronomy? • The investigation of the the universe or cosmos.

2. Lecture Series 1: Scale • What do scientist mean by scale? • Class provide Dr. Connolly with an answer. • How fast does light travel? • 300, 000 km/s • What is a light year? • The distance light travels within one standard Earth year. How long is that, be exact? • 365.256 mean solar days - 1 Ly

3. Lecture Series 1: Scale • How far does light travel in a year? • One light year = 10 trillion kilometers • That is ~ 6 trillion miles, for those who need to think in miles.

4. Lecture Series 1: Distance • 1 Ly or light year = the distance light travels in one standard Earth year, or 10 trillion km or 6 trillion miles. • If the distance to the nearest star is 4 Ly, than 4 x 10 trillion km = 40 trillion km.

5. Lecture Series 1: Distance • What is a parsec? • An astronomical unit of distance that is = 3.262 Ly • The term parsec is derived from “parallax in arc seconds” • This can be related by the equation • Distance (in parsec) = 1/parallax (in arc seconds)

6. Lecture Series 1: Distance • Another unit of measurement for distance in the cosmos is based on triangulation. To find the distance to a point in two-dimensional space, one needs three points. This come from the discipline of trigonometry. • This process forms the bases of Parallax.

7. Lecture Series 1: Distance

8. Lecture Series 1: Distance • Parallax = apparent displacement of a foreground object relative to the background of an observer. • The closer the object the larger the parallax.

9. Lecture Series 1: Distance • Astronomers also use angular measurement. This is not always an easy system to think within and has nothing to do with time. • One circle contains 360o of measurement. Within that are 180o, 90o, 45o etc. with 180o = to the horizon. • 1o can is subdivided into fractions of degrees termed arc minutes. There are 60 arc minutes (60’) and an arc minute is further divided into 60 arc seconds (60”). The Sun and Moon have an angular size of 30 arc minutes of the horizon. • Thus 1/60 of a degree = 1 arc minute (60’) • Thus 1/3600 of a degree - 1 arc second (60”) • Arc seconds are really small! Angular size depends on the actual size of an object and its distance.

10. Lecture Series 1: Distance • Astronomers also use a system of celestial coordinates to determined positions of objects in the cosmos. • Declination is similar to latitude, units = degrees • Right ascension is similar to longitude, units = time

11. Lecture Series 1: Distance • Within the Solar System or smaller areas of the Universe, how is distance determined or measured? • The easiest unit is the Astronomical Unit or AU. • 1 AU is defined as the distance from the Earth to the Sun. That is ~ 150,000,000 km or ~ 93,000,000 miles. • There are ~ 63,240 AU in 1 Ly or light year.

15. Lecture Series 1: Constellations • What are constellations? • A creation of the human mind. • Interesting but not important to our voyage through the cosmos.

16. Lecture Series 1: Earth’s Movement • Celestial Sphere • No the stars are not attached to the Earth. • But the Earth does rotate around an axis ( tilt = 23.45o), and around a star (tilt = 0.01o). • Celestial equator

17. Lecture Series 1: Earth’s Movement • Daily Changes • These are due to the fact that the Earth rotates. Time is determined by the Earth’s rotation around its axis. • Solar Day = From Noon to Noon = 24 hr • Sidereal Day = The time it takes for a star or constellation to complete one cycle in the sky, returning to the same place it was observed the previous day. (sidus which is Latin = star) 23hr 56min.

18. Lecture Series 1: Time • Solar vs. Sideral Day • Earth revolves around the Sun (30 km/s) in 365.256 days, but around its axis (360o; 0.5 km/s) in 24 hr. In one day the Earth rotates 360o AND moves through space.

19. Lecture Series 1: Earth’s Movement & Time • How long is a standard solar year? • This is known as a sidereal year, which is ~365.256363051 solar days. • It does change ever so slightly from year to year. • What is one tropical year? • From one vernal equinox to the next, ~365.242 solar days. • This is what our calendars are based upon - Gregorian. • If not, seasons would change!

20. Lecture Series 1: Earth’s Movement & Seasons • Why does Earth experience seasons? • Simple, the Earth is tilted AND it revolves around the Sun. • Seasons result from the changing height of the sun above the horizon. • Ecliptic = apparent motion of the Sun in the sky- Earth’s orbit around the Sun. • What are the important dates and names for seasons?

21. Lecture Series 1: Earth’s Movement & Seasons • Ecliptic = apparent motion of the Sun in the sky- Earth’s orbit around the Sun. • = A geometric plane within our solar system that contains the orbit of the Earth and most other planets around the Sun. • = An imaginary plane that contains the Earth’s orbit around the Sun. • = Through the course of a standard Earth year the Sun’s path as seen from Earth lines within a plane.

22. Lecture Series 1: Earth’s Movement & Seasons

25. Lecture Series 1: Earth’s Movement & Seasons • The two points where the ecliptic intersects the celestial equator are known as equinoxes. • At these places night and day on Earth are the same duration. Such dates are known as Spring (vernal) and Autumn (autumnal) ! • In the Christian Church Spring = Eostar and Autumn = Mabon

26. Lecture Series 1: Earth’s Movement & Seasons • The point on the ecliptic where the Sun is at its northernmost point above the celestial equator is known as the summer solstice (sol which is Latin = sun and stare which is Latin = to stand) . • In the Northern hemisphere it is the longest day of the year. • Known as Litha in Christian Church.

27. Lecture Series 1: Earth’s Movement & Seasons • The point on the ecliptic where the Sun is at its southernmost point below the celestial equator is known as the winter solstice. • In the Northern hemisphere it is the shortest day of the year. • Known as Yule in the Christian Church.

28. A Few Historical Facts • From Celtic Calendars the following names were used and evolved with time. • Imbolic or Oimeaig = the beginning of Spring which is ~ Groundhog Day (Church = Candlemas). • Beltane = Spring • Lughnasadh = Autumn (Church Lammas) • Samhain = Winter and the new year (Church = Hollowmas)

29. Lecture Series 1: Earth’s Movement & Seasons • The 12 constellations that the Sun passes as it moves along the ecliptic are known as the zodiac. These were of important to astrologers of old. • In other words, the reason the stars and constellations change with the season is because the cosmos moves as we do through it.

30. Lecture Series 1: Earth’s Movement & Facts • When the Earth is closest to the Sun, it is termed the perihelion (~ 2 January). • When the Earth is farthest from the Sun, it is termed the aphelion (~2 July).

31. Lecture Series 1: Earth’s Movement & Lunar • The Moon has phases or changes in appearance during the night sky because of how the light from the Sun reflects off the Moon as it rotates around the Earth. • New Moon, Full Moon, etc.

32. Lecture Series 1: Earth’s Movement & Lunar

33. Lecture Series 1: Earth’s Movement & Lunar • Sidereal month = 27.32 Earth days. One complete revolution around Terra. • Synodic month = 29.53 Earth days. One complete cycle of phases ---full moon to full moon.

34. Lecture Series 1: Earth’s Movement & Eclipses • A Lunar eclipse occurs when the Earth comes between the Moon and our star. The alignment is perfect or nearly thus. Star light no longer fully illuminates the surface of the Moon. These can be partial or total. • Such eclipses, when the Moon passes through the Earth’s shadow, last ~ 100 mins. Such an eclipse is visible from all locations on the Earth’s night side.

35. Lecture Series 1: Earth’s Movement & Eclipses • A solar eclipse occurs when our satellite, the Moon, passes directly in front of the Sun. They are very short in time, no more than 7 mins for total cover. • During a total eclipse of the Sun, the Moon is PERFECTLY aligned between Earth and the Sun (known as Umbra). • During a partial eclipse, the Moon is not PERFECTLY aligned (known as Penumbra).

36. Lecture Series 1: Earth’s Movement & Eclipses • Because the Moon’s orbit around Earth is not perfectly circular, it moves far enough away from the Earth that the blockage is not always complete, as in a total eclipse. Such an eclipse is known as a Annular eclipse.

39. Additional Readings • I’ll have to think about this issue!