Observing the PlanetsObserving the Planets

Our Place in the UniverseOur Place in the Universe

Imagine each star is a grain of Imagine each star is a grain of sand….sand….

• All the visible stars would fill a All the visible stars would fill a thimblethimble

• The 200 billion stars in the Milky The 200 billion stars in the Milky Way Galaxy would be contained in a Way Galaxy would be contained in a large wheelbarrowlarge wheelbarrow

• There is not enough sand on all There is not enough sand on all the beaches on Earth to represent the beaches on Earth to represent the entire universe (over 100 billion the entire universe (over 100 billion galaxies).galaxies).

Observing the PlanetsObserving the Planets

Our Place in the Universe…2Our Place in the Universe…2

Think of the universe in vast distances…and a Think of the universe in vast distances…and a measuring stick of light which travels 300 000 km/s.measuring stick of light which travels 300 000 km/s.

• Our Moon is one light-SECOND away.Our Moon is one light-SECOND away.

• The planet Jupiter is 0.5 light-HOURS away.The planet Jupiter is 0.5 light-HOURS away.

• Former planet Pluto is five light-HOURS from Earth.Former planet Pluto is five light-HOURS from Earth.

• Our nearest star, Alpha Centauri, is four light-YEARS Our nearest star, Alpha Centauri, is four light-YEARS away.away.

• The distance from the Sun to the centre of the Milky The distance from the Sun to the centre of the Milky Way Galaxy is 24 000 light-years.Way Galaxy is 24 000 light-years.

• The Milky Way Galaxy is 2.3 million light-years from The Milky Way Galaxy is 2.3 million light-years from the Andromeda Galaxy, our nearest galactic neighbour.the Andromeda Galaxy, our nearest galactic neighbour.

Observing the PlanetsObserving the PlanetsThe PlanetsThe Planets

Like our Moon, the planets shine be reflected sunlight.Like our Moon, the planets shine be reflected sunlight.

Although the planets are much farther away than the Although the planets are much farther away than the Moon, they are still in our celestial neighbourhood. Moon, they are still in our celestial neighbourhood. Thus, planets appear brighter than stars.Thus, planets appear brighter than stars.

So brightness will determine whether you are looking So brightness will determine whether you are looking at planet. at planet.

What else?What else?

Stars twinkle, while planets do not. Stars are points of Stars twinkle, while planets do not. Stars are points of light very, very far away. The beam of light that we can light very, very far away. The beam of light that we can see is very thin, and the beam is easily disrupted by see is very thin, and the beam is easily disrupted by turbulence in Earth’s atmosphere. Planets are closer to turbulence in Earth’s atmosphere. Planets are closer to Earth, and thus, the beam of light is larger and less Earth, and thus, the beam of light is larger and less susceptible to atmospheric turbulence. susceptible to atmospheric turbulence.

So, a bright and non-twinkling visible dot in the night So, a bright and non-twinkling visible dot in the night sky is likely a planet.sky is likely a planet.

Anymore ideas?Anymore ideas?

Observing the PlanetsObserving the Planets

The Planets…2The Planets…2

The planets orbit around the Sun in the same direction The planets orbit around the Sun in the same direction AND within the same flat ecliptic plane. AND within the same flat ecliptic plane.

Why flat and ecliptic? Why flat and ecliptic?

During the solar system’s formation about 5 billion During the solar system’s formation about 5 billion years ago, the Sun and its family of planets arose from years ago, the Sun and its family of planets arose from a flat cloud of dust and gas. That shape was retained.a flat cloud of dust and gas. That shape was retained.

The planets appear to move within Earth’s ecliptic.The planets appear to move within Earth’s ecliptic.

The planets move slowly and their positions change The planets move slowly and their positions change only slightly from night to night. They rise in the east only slightly from night to night. They rise in the east and set in the west as part of the sky's diurnal motion.and set in the west as part of the sky's diurnal motion.

Diurnal Motion?Diurnal Motion?

The perceived movement of the star, planets, Moon and The perceived movement of the star, planets, Moon and Sun as they move relative to our viewing point.Sun as they move relative to our viewing point.

Observing the PlanetsObserving the PlanetsThe StarsThe Stars

Stars have relatively fixed positions with respect to Stars have relatively fixed positions with respect to each other (i.e., the constellations do not change each other (i.e., the constellations do not change with time).with time).

Yet, the stars do move. Their motion is only Yet, the stars do move. Their motion is only noticeable to the unaided eye after a long time, noticeable to the unaided eye after a long time, perhaps tens of thousands of years or more.perhaps tens of thousands of years or more.

Nearby stars also exhibit parallax, but this is only Nearby stars also exhibit parallax, but this is only visible in a telescope.visible in a telescope.  

Stars occur at different distances from the Earth.Stars occur at different distances from the Earth.

Next, the Earth rotates once a day. We cannot detect Next, the Earth rotates once a day. We cannot detect this motion so it appears to us that the stars, the this motion so it appears to us that the stars, the Sun, the Moon and the planets rotate around us (i.e., Sun, the Moon and the planets rotate around us (i.e., they rise in the east and set in the west, once a day.) they rise in the east and set in the west, once a day.) This is called diurnal motion.This is called diurnal motion.

Observing the PlanetsObserving the PlanetsSeeing PlanetsSeeing Planets

Venus, Mars, Jupiter and Saturn can be seen with the Venus, Mars, Jupiter and Saturn can be seen with the unaided eye, although it is not possible to see detail unaided eye, although it is not possible to see detail without a telescope.without a telescope.

Each planet moves by a fixed schedule…and thus, it Each planet moves by a fixed schedule…and thus, it is visible by that schedule. On a given evening, a is visible by that schedule. On a given evening, a planet may be visible the entire evening, just part of planet may be visible the entire evening, just part of the evening or not at all. The presence of a planet the evening or not at all. The presence of a planet changes very little over the course of an evening or changes very little over the course of an evening or two, but it will change over a period of weeks or two, but it will change over a period of weeks or months. months.

When Venus is visible, it is much brighter than When Venus is visible, it is much brighter than anything in the sky with the exception of the Sun and anything in the sky with the exception of the Sun and Moon. Moon.

The brightness of a planet may change over The brightness of a planet may change over extended periods. extended periods. When Mars is bright, it is bright When Mars is bright, it is bright red; when it is dim, it is a pale pink. This change red; when it is dim, it is a pale pink. This change takes two years. takes two years.