Download Observing the Planets

Observing the Planets
Our Place in the Universe
Imagine each star is a grain of
sand….
• All the visible stars would fill a
thimble
• 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).
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.
• The planet Jupiter is 0.5 light-HOURS away.
• Former planet Pluto is five light-HOURS from Earth.
• 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 Milky Way Galaxy is 2.3 million light-years from
the Andromeda Galaxy, our nearest galactic neighbour.
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?
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.
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.
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.