1. Observing the Planets Our Place in the Universe Imagine each star is a grain of sand…. All the visible stars would fill a thimble All the visible stars would fill a thimble The 200 billion stars in the Milky Way Galaxy would be contained in a large wheelbarrow The 200 billion stars in the Milky Way Galaxy would be contained in a large wheelbarrow There is not enough sand on all the beaches on Earth to represent the entire universe (over 100 billion galaxies). There is not enough sand on all the beaches on Earth to represent the entire universe (over 100 billion galaxies).

2. Observing the Planets Our Place in the Universe…2 Think of the universe in vast distances…and a 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 away. Our nearest star, Alpha Centauri, is four light-YEARS away. The distance from the Sun to the centre of the Milky Way Galaxy is 24 000 light-years. The distance from the Sun to the centre of the Milky Way Galaxy is 24 000 light-years. The Milky Way Galaxy is 2.3 million light-years from the Andromeda Galaxy, our nearest galactic neighbour. The Milky Way Galaxy is 2.3 million light-years from the Andromeda Galaxy, our nearest galactic neighbour.

3. Observing the Planets The Planets Like our Moon, the planets shine be reflected sunlight. Although the planets are much farther away than the Moon, they are still in our celestial neighbourhood. Thus, planets appear brighter than stars. So brightness will determine whether you are looking at planet. What else? Stars twinkle, while planets do not. Stars are points of light very, very far away. The beam of light that we can see is very thin, and the beam is easily disrupted by turbulence in Earth’s atmosphere. Planets are closer to Earth, and thus, the beam of light is larger and less susceptible to atmospheric turbulence. So, a bright and non-twinkling visible dot in the night sky is likely a planet. Anymore ideas?

4. Observing the Planets The Planets…2 The planets orbit around the Sun in the same direction AND within the same flat ecliptic plane. Why flat and ecliptic? During the solar system’s formation about 5 billion years ago, the Sun and its family of planets arose from a flat cloud of dust and gas. That shape was retained. The planets appear to move within Earth’s ecliptic. The planets move slowly and their positions change only slightly from night to night. They rise in the east and set in the west as part of the sky's diurnal motion. Diurnal Motion? The perceived movement of the star, planets, Moon and Sun as they move relative to our viewing point.

5. Observing the Planets The Stars Stars have relatively fixed positions with respect to each other (i.e., the constellations do not change with time). Yet, the stars do move. Their motion is only noticeable to the unaided eye after a long time, perhaps tens of thousands of years or more. Nearby stars also exhibit parallax, but this is only visible in a telescope. Stars occur at different distances from the Earth. Next, the Earth rotates once a day. We cannot detect this motion so it appears to us that the stars, the Sun, the Moon and the planets rotate around us (i.e., they rise in the east and set in the west, once a day.) This is called diurnal motion.

6. Observing the Planets Seeing Planets Venus, Mars, Jupiter and Saturn can be seen with the unaided eye, although it is not possible to see detail without a telescope. Each planet moves by a fixed schedule…and thus, it is visible by that schedule. On a given evening, a planet may be visible the entire evening, just part of the evening or not at all. The presence of a planet changes very little over the course of an evening or two, but it will change over a period of weeks or months. When Venus is visible, it is much brighter than anything in the sky with the exception of the Sun and Moon. The brightness of a planet may change over extended periods. When Mars is bright, it is bright red; when it is dim, it is a pale pink. This change takes two years.