December 4th 2002
The above graphic shows the
position of Venus in the sky over the next few months at 6am
every morning. Notice how it is rising in the sky every day
until around the end of December. This all corresponds to
the orbit of Venus and how it is currently racing away from
us and eventually how it loops back around the far side of
the Sun. See the full explanation of this below.
If you do not know what it is - seeing
the brilliant morning show put on by Venus can be quite surprising,
puzzling or even unsettling to some.
Ever since I started getting involved
with the OCA and especially attending Outreach events I have
been quite surprised at times by the questions that I get
asking me what the bright star is over in the west after sunset
- or over towards the east very early in the morning.
While most people have heard of the
term "the Morning Star" and "the Evening Star"
many do not realize this is not referring to a star (or a
couple of them even) at all - but that it is in fact Venus.
Below is an extract from an email
sent to me recently by one very curious Irvine resident.
my question is about something
I see in the sky here in Orange County as the sun begins
to rise. I've seen this thing every morning for the past
couple of months. It appears low in the sky and just to
the right of the peak of Saddleback Mountain and is a diagonal
slash of white, except once a couple of weeks ago when the
sunrise began it took on a glowing red and gold color, then
became white as usual. It fades and is no longer visible
once the sun is up. Do you know what this is?
This was my reply
suspect what you are seeing is the planet Venus.
This is often referred to as "the Morning Star"
and also "the Evening Star". In the past few months
Venus has switched from being visible in the evening - to
being visible in the morning. This is due to it swinging
around in front of the Sun as it moves in it's orbit. When
it gets closer and closer to us the planet will actually
get bigger and bigger and brighter and brighter. If you
were to look at it using a telescope you would see it change
in phase just like the moon does. At some point it would
be more of a slash (line) of white than a round object like
you might expect.
change in color you describe is attributable to atmospheric
disturbances due to how close the object is to the horizon
(notice that most stars twinkle far more when they are lower/closer
to the horizon).
hope that helps.
The reason someone might see the brilliant
Venus as more of a slash than a star can be attributed to
perhaps very acute vision and being able to perceive the "star"
is in fact more of a crescent disk - although it could also
be related to simply how bright it is.
As for that color aberration - one
"general" rule often quoted by astronomers about
planets is that they very rarely twinkle like stars often
do. In fact this is often a very easy test anyone can do to
determine if a bright "star" they see in the sky
at night is in fact a planet.
Stars twinkle because they are so
incredibly distant from us that any disturbances in the atmosphere
will bounce around the "ray" of light reaching your
eye - effectively making the star twinkle. This is more pronounced
the closer the star is to the horizon (more atmosphere to
Planets don't "suffer" from
this twinkling anything like stars because, even though they
are still very far away, they still are represented in the
sky as a definite disk rather than a point source of light.
Therefore on average what you will find is that these "multiple"
rays of light from the planet (actually just reflected sunlight
in fact) manage to cancel out a lot of the atmospheric disturbances.
In the case above Venus was probably
so much closer to the horizon - and the atmospheric conditions
such that this general rule about non-twinkling planets just
didn't hold. Oh well - the exception that proves the rule!
Venus is of course the closest planet to
us - it is also covered in clouds. For this reason it reflects
the Sun very well making Venus the brightest of all the planets.
However throughout the orbit Venus gets progressively closer
and then further away from us - and during this time it undergoes
a very distinct change is phase and size. The images below
illustrate this effect very well.
Showing the position of the
Earth and Venus on Nov 22nd.
Notice how close Venus is to
the Earth. That contributes to it looking so large in
a telescope - it's position also results in the very
highly crescent shape seen above.
By December 15th Venus is
a little further away.
Notice how Venus is getting
smaller in our telescopes view and how it is becoming
less of a crescent.
By February 1st Venus is much
further away from us.
Now it looks much smaller and is like a "half
moon" phase to us.
All the above images were created
using Starry Night Pro version 3. This is a planetarium program
available from Space.com.
More information at StarryNight.com