1. The Solar System
Ancient observers noticed that five bright objects among the stars at night.
They called these objects planets, from the Greek word meaning “wandering star,” and named them Mercury, Venus, Mars, Jupiter, and Saturn.

2. The Solar System
Today, we define the solar system as the sun and all objects that are gravitationally bound to the sun.
The solar system is divided into the inner planets (Mercury, Venus, Earth, and Mars) and the outer planets (Jupiter, Saturn, Uranus, and Neptune)
The dwarf planet Pluto is the oldest known member of a smaller group of frozen worlds orbiting beyond Neptune.

3. Orbits
An orbit is a regular, repeating path that an object in space follows around another object.
An object in orbit is called a satellite.

4. Orbits
The orbits of the planets are slightly elliptical but almost circular.
The Sun is at a point called the focus that is offset from the center of the orbit.

5. Motion of Planets
In addition to orbiting around the Sun, the planets also rotate.
An axis is the imaginary line that passes through the center of a planet from pole to pole.

6. One astronomical unit (AU) is equal to 150 million kilometers - or the distance from Earth to the Sun.

7. The Planets The planets are commonly classified in two groups:
The terrestrial planets include Mercury, Venus, Earth, and Mars.
The gas giants include Jupiter, Saturn, Uranus, and Neptune.

8. Mercury
Mercury - the closest planet to the sun, is the second smallest (after Pluto) in both size and mass.
Mercury has no moons.
Length of day = 59 Earth Days
Length of year = 89 Earth days

9. Venus Venus - appears as the brightest planet in the evening sky.
Venus has no moons.
It was named after the Roman goddess of love.

10. Earth
Earth - is a small, rocky planet with an atmosphere that is made of mostly nitrogen (78 percent N2) and oxygen (21 percent O2).
It has one moon.

11. Mars
Mars - The fourth planet out from the sun. It appears as a reddish point of light in the night sky.
It is named after the Roman god of war.
It has 2 moons.

12. Jupiter
Jupiter - The fifth planet from the sun. It is the largest of the planets in our solar system.
Jupiter’s mass is greater than the combined mass of all of the other planets.
With 63 known moons, Jupiter is like a mini solar system. It also has very faint rings.

13. Saturn
Saturn - about 10 times the size of Earth. It is the second largest planet in our solar system.
The most amazing feature of Saturn is its rings. Like Jupiter, it has many natural satellites (moons)

14. Uranus
Uranus - The seventh planet from the sun. Uranus can barely be seen without a telescope and was discovered in 1781.
It has 27 moons, plus rings.

15. Neptune
Neptune - the eighth planet from the sun. It is the outermost of the gas planets.
It was discovered in 1846.
It has 13 moons plus rings.

16. Other Parts of our Solar System
Pluto - is a dwarf planet.
Discovered in 1930, Pluto was named for the Roman god of the underworld.
Outside the orbit of Pluto is a region called the Kuiper Belt.
The Kuiper Belt stretches to 1,000 AU and is contains many asteroid-size objects.

17. Asteroids
An asteroid is an object that orbits the sun but is too small to be considered a planet.
The asteroid belt is located between Mars and Jupiter.
The largest asteroid, named Ceres, is 933 kilometers (580 miles) across.

18. Comets Comets are made mostly of ice and dust.
Comets revolve around the Sun in elliptical orbits.
The inner core of the comet is the nucleus.
As a comet gets closer to the Sun, it forms a tail.

19. Meteors and Meteorites
Occasionally, chunks of rock or dust break off from a comet or asteroid and form a meteor.
As Earth orbits the sun, it passes through this debris, creating a meteor shower as the small bits burn up in the atmosphere.

20. Meteors and Meteorites
If a meteor is large enough to survive the passage through Earth’s atmosphere and strike the ground, it becomes a meteorite.

21. The Earth
Earth’s shape is spherical except for a slight bulge at the equator.
The Sun is not at the center of Earth’s orbit. Due to this, the distance from Earth to the Sun changes as Earth revolves.

22. The Moon The Moon is about one quarter the size of Earth.
Because of the Moon’s small mass, its gravity does not attract an atmosphere.
It is a distance of 385,000 kilometers from Earth.
The moon completes one orbit every 29 days.

23. The Moon The same side of the moon faces Earth at all times.
This does not mean that the Moon does not rotate. It just rotates more slowly than Earth.

24. The Moon
Craters are large, round pits that cover much of the Moon’s surface.
Brighter areas are called highlands because they are higher in elevation.
Maria are low, dry areas that were flooded with molten lava billions of years ago.

25. Tides
The daily cycle of rising and falling ocean levels is called a tide.

26. Moon Phases
The gradual change in the appearance of the Moon is called the lunar cycle.
The lunar cycle occurs because of the relative positions of Earth, the Moon, and the Sun.
Although the lunar cycle is a continuous process, there are eight recognized phases.

28. The orbit of the Moon is tilted about 5 degrees from Earth’s orbit.
The lunar cycle is caused by the angle the Moon makes with Earth and the Sun as it orbits Earth.

29. The Sun-facing side of the Moon is lit by sunlight almost all the time.
The lighted side of the Moon faces away from Earth.

30. Lunar Eclipse
A lunar eclipse occurs when Earth’s shadow falls on the Moon.
Most months the Earth’s shadow does not block the sunlight from hitting the Moon because of its tilted orbit.
A lunar eclipse can be total or partial and all observers on the dark side of Earth can see it at the same time.
A partial eclipse occurs when only part of the Moon falls in Earth’s shadow.

31. Solar Eclipse
A solar eclipse occurs when the Moon’s shadow falls on Earth.

32. A total solar eclipse is caused by the Moon’s shadow blocking out the Sun.
The Sun is completely blocked by the Moon’s shadow for about 2-3 minutes.

33. The Seasons
The seasons are caused by the 23-degree tilt of Earth’s axis of rotation.

34. During summer in the Northern Hemisphere, the north end of the axis tilt is facing toward the Sun.
This results in more direct sunlight and higher temperatures.

35. The Sun
The Sun is a star. A star is a giant, hot ball of gas held together by gravity.
The Sun is medium-sized star compared with other stars in the universe.

36. The corona is the outermost layer of the sun’s atmosphere.
Sunspots are areas of gas that are cooler than the gases around them.

37. Auroras – (sometimes called the northern lights) occur when layers of our atmosphere are energized by solar winds.

38. Galaxies
A galaxy is a huge group of stars, dust, gas, and other objects bound together by gravitational forces.
Our galaxy, the Milky Way, contains billions of stars. It is a spiral galaxy.

39. Types of Galaxies
The Milky Way is a flattened, rotating system that contains young to middle-aged stars, along with gas and dust. It is classified as a spiral galaxy

40. There are 4 types of Galaxies:
Spiral galaxies consist of a central, dense area surrounded by spiraling arms.
Barred spiral galaxies have a bar-shaped in the center.
Elliptical galaxies look like the center of a spiral galaxy without the arms.
Lenticular galaxies are lens-shaped.

41. Stars
A constellation is a group of stars that form a pattern when seen from Earth.
The Big Dipper constellation is shown below.

42. Stars come in a range of sizes and temperatures.
Example: Our Sun is a medium-sized star.

43. Size of Stars
Stars that are smaller than the sun come in two main categories:
1. dwarfs stars
2. neutron stars.
Example - Sirius “The Dog Star” - is the largest known white dwarf.

44. Temperature of Stars
If you look closely at the stars on a clear night, you might see a slight reddish or bluish tint to some stars.
This is because their surface temperatures are different.
The color of light is related to its energy.

45. Temperature and Color of Stars

46. A protostar is the earliest stage in the life cycle of a star.
A star begins its life inside a huge cloud of gas (mostly hydrogen) and dust called a nebula.
A protostar is the earliest stage in the life cycle of a star.
The Orion Nebula was the birthplace of these protostars.