One of the challenges many new to
Astronomy face is knowing what they are seeing. A friend publishes an excellent
and helpful group called ‘Our Wide Sky” There are helpful tips and good friends
to be found at
www.facebook.com/groups/OurWideSky/about/
When we first start exploring the
sky we know that there are stars and planets out there, but how do we know
which is which. In this beginner tip we’ll talk about the difference between
stars and planets.
There are 5 planets that you can
see with your naked eye: Mercury, Venus, Mars, Jupiter and Saturn. The other
planets are too dim for us to see. You can actually see Earth, but you look
down to see that :).
With your naked eye, over a year,
and with a dark sky, you can see around 9000 stars throughout the year. So how
do you pick out any of the 5 planets among all those stars. Here are some tips:
If it is not on the ecliptic it
is not a planet. The Ecliptic is the path the Sun takes through your sky. Where
this is will depend on your latitude and, because the Earth spins at an angle
to the plane of orbit around the Sun, it also depends on the time of year. The
path of the Sun through the sky is the plane of the solar system, because all
of the planets orbit the sun in approximately the same plane. This means that
if you are going to see a planet it will be on this plane, on the Ecliptic.
Venus and Mercury don’t stay up
late. Because Venus and Mercury both orbit the Sun inside the orbit of the
Earth, we see them only in proximity to the Sun. They will be in the East
around Sunrise, or the West around Sunset. They don’t rise high in the night
sky, and they aren’t up all night.
Mars, Jupiter and Saturn are
bright. They are usually among the brightest things in the night sky. Sirius
and Canopus are the brightest stars, Jupiter is always brighter than Sirius.
Mars will be brighter at opposition (when it is on the opposite side or Earth
from the Sun). Saturn at opposition is around as bright as the fourth brightest
star in the night sky.
Stars are points, planets are
disks. This is really hard to pick with the naked eye, but with practice you’ll
be able to get it right most of the time. With binoculars you should be able to
see the difference, a planet looks like it has a flat surface. Stars are just
points of light. Sometimes the atmosphere can play tricks, but again, practice,
observation and confirmation will help you identify the difference.
Planets move differently. This
takes time to observe. The word planet is from the ancient Greek word for
“”wanderer””. The movement of stars happens over millenia, but the movement of
planets happens over months, this is because we are orbiting the Sun at
different rates to the planets so we see them move relative to the background
stars. There is also a part of the year when they move backward across the sky,
this is called retrograde motion.
Imaging you are on a train and
following another train on a different track. Your train is travelling faster.
While you are behind the train it appears to move with you trough the landscape.
When you come up beside it it seems to stand still relative to you, and when
you pass it it seems to go backward for a time. Solar systems are more complex
than two trains, but the analogy should help.
Retrograde motion was one of the
most difficult things for early astronomers to model. They thought that the
Earth was the centre of the Universe, and retrograde motion did not make sense
in this model. It works perfectly in a solar system model.
Using these characteristics you
should be able to see a bright object and determine whether it is a star or a
planet.”