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Although we will not talk about the Dawn mission in this issue of AstroNews, we will talk about the name: Dawn. The meaning of dawn that you are probably
familiar with is something like "Just before sunrise when the horizon starts to lighten". It may surprise you but an exact definition of dawn exists. Not
only that, but more than one type of dawn is defined.
The three different dawn definitions have different names. You have "Civil dawn", "Nautical dawn", and "Astronomical dawn". The one thing they all have in common
is that the Sun is still below the horizon, but they differ in the definition of how much. Here is the full definition of each:
Astronomical dawn is the time at which the sun is 18 degrees below the horizon in the morning. Astronomical dawn is that point in time at which the
sun starts lightening the sky. Prior to this time, the sky is completely dark.
Nautical dawn is the time at which the sun is 12 degrees below the horizon in the morning. Nautical dawn is defined as that time at which there
is just enough sunlight for objects to be distinguishable.
Civil dawn is the time at which the sun is 6 degrees below the horizon in the morning. Civil dawn is defined as that time at which there is enough
light for objects to be distinguishable and that outdoor activities can commence.
Currently the two Mars rovers, Spirit and Opportunity, are still roving around the planet and are alive and well. Of course things are slowly breaking
down because they were not designed to operate this long. But NASA is doing everything to stretch the missions because the rovers are still returning
Next to the rovers, the Mars Reconnaissance Orbiter is also producing also amazing results. The highest resolution images come from this mission and we are
seeing the red planet in more detail than ever. But there are still more missions. Mars Express and Mars Odyssey are two other missions
that are currently operational, but that's not all. On June 30th we will see the launch of yet another mission to Mars. This mission is called
"Dawn", and in August NASA will launch the "Phoenix" mission. It's going to busy around our red neighbor! Next month we will explain about these two
missions in more detail, so stay tuned!
Each planet in our solar system goes around the Sun in a different amount of time than the Earth.
Of course a year is the time it takes for a planet to go all the way around the Sun once. So, each planet's
year is shorter or longer than that of the Earth. The Earth takes about 365.25 days to go around the Sun once.
(Note: we will just talk about Earth days here. Each planet also has its own length of day, so things could get a bit
confusing if we try to take on too much at once.) The planets closer to the Sun have a shorter year than the Earth,
while the ones further from the Sun have a longer year.
Here is a chart showing (in Earth years) how long it takes each planet to go all the way around the Sun once:
|Planet||Length in Earth Years
|Mercury|| 0.241 yrs
|Venus|| 0.615 yrs
|Earth|| 1.000 yrs
|Mars|| 1.881 yrs
|Jupiter|| 11.86 yrs
|Saturn|| 29.46 yrs
|Uranus|| 84.01 yrs
|Neptune|| 164.8 yrs
|Pluto|| 248.6 yrs
Consider this: you would be a different age on the other planets.
I have a friend who recently turned 40. That's 40 in Earth years. She would already be 65 in years on
Venus, but still just 21 years old on Mars - and less than 4 years old on Jupiter. So, take your pick:
retired, in college, or still a pre-schooler with these extra-terrestrial ages. Using the chart above and the
following formula, you can easily calculate your own age in each planet's years:
X = Your current age
Y = Any number from the chart above
X / Y = Your age in another planet's years
Are you an old wo/man on Mercury? Still an infant on Saturn?
As you know, our Sun has a yellow color, but a lot of other stars (which are suns as well) have completely different colors.
Betelgeuse for example, in the constellation Orion, is red. In the same constellation however, there are several stars that are
much bluer then our Sun. Look at the image below, and notice the different colors of stars. How come the difference?
Constellation of Orion
The cause of these color differences is temperature. Every star is different, just like every human being is different, and
different stars have different temperatures. Maybe surprisingly a red star is a lot cooler then a blue or white star. But we
can understand it with the following example. If you take a piece of iron and hold it in a flame, it will become red-hot. It
indeed will start to glow red. But when we make it even hotter, it will become a lighter color, more towards yellow. After
yellow it will become white-hot.
The same principle holds true for stars. Our Sun has a surface temperature of about 6000 Kelvin (10,340 F) but Betelgeuse has
a surface temperature of 3650 Kelvin (6110 F). Rigel however, another star in the constellation of Orion, has a surface degree
of 11,000 Kelvin, that is almost twenty thousand degrees Fahrenheit!
As you can see from the calendar this month, there is not much spectacular going on in the sky this month. Even the Space Shuttle
launch that was scheduled for early April has been postponed to a later date, because the external fuel tank was damaged by hail
last month. The Space Shuttle will most likely launch somewhere early next month.
But thankfully, there is CenTex! Centex astronomy weekend is an event organized by our friends from the Austin Astronomical
Society (AAS). This annual starparty is held at the Canyons of the Eagles state park on
Lake Buchanan. AAS has a nice observatory there, and during this weekend the observation field is open to the public. You will have
the chance to look through dozens of telescopes. Additionally, there will be lectures about astronomy.
Centex is held on April 20-21, and is free and open to the public. For more information and driving directions, please check
Last month we talked about the fact that gravity exists everywhere. But if this is true, how can an astronaut float in space
like there is no gravity at all? Obviously something is different up there! Well, not exactly. It is not about where the
astronauts are, but more about how they are moving relative to Earth. If the circumstances are right, you can be weightless
right here on Earth!
In both cases you experience 1G gravity in the elevator
Einstein came up with a good explanation, and it is still used today to explain his theory of general relativity to students.
It is the famous “elevator” example. Imagine you are in an elevator (see illustration on the left). The doors of the elevator
are closed, and there are no windows in the elevator. In case A, the elevator is on Earth, and does not move. You would experience
a gravity of one G, just as you would expect.
But elevator B is in deep space, in an area where there is no gravity (although we know this does not exist, for this example we
imagine that it does). This elevator is accelerating because we strapped a (completely silent) engine to it. The acceleration is
equal to one G. The interesting fact is, a passenger inside elevator A or B would be unable to tell the difference between the
two — the motionless elevator on Earth and the moving elevator in space would feel exactly the same to the people inside them.
Similarly, look at the illustration on the right. Case A is bad news, because now the elevator is crashing down, or, to say it
properly, is in “free fall.” During this time the passenger will be weightless. Case B is the imaginary place again where no
gravity exists, and the passenger is truly weightless. Again, a passenger would be unable to tell the difference between the
In both cases you experience no gravity in the elevator
This is exactly what happens with the astronauts. They aren’t really in an area without gravity — they are in constant free
fall around Earth! Therefore they experience a zero-G environment. NASA even operates planes that simulate zero-G. The plane flies
in an up, down, up pattern over and over. Every time the plane dives down, passengers are weightless for about 25 seconds, and that
is not even in space!
The first special event of the month is the lunar eclipse of March third. Although not as spectacular as a solar
eclipse, this event is still something to pay attention to. In this case, we will not be able to see the start of
the eclipse, because at the time the Moon rises, it will already be in full eclipse. This probably will make
things even more spectacular, because now the Moon will be dramatically red when it rises. Another benefit of a
lunar eclipse over a solar eclipse is that it is completely safe to watch. The Moon will not blind you as the Sun
does. Also the time is perfect for observation, because when the Moon is full, it rises at the exact same time the
Sun sets. For Saturday this will be at 6:31 pm.
The second special Event of March is the vernal equinox. The vernal equinox is March 20 this year, at precisely 7:07
p.m. CDT. The word “equinox” is Latin for equal night. That means that on the equinoxes, both spring and fall, the
length of night and day are equal. But do you know why?
Over the course of a year, the motion of the Sun and planets appear to trace out a path across the sky. That path is
called the “ecliptic.” Earth’s equator projected onto the sky is called the “celestial equator.” There are two points
where the ecliptic crosses over the celestial equator. When the Sun reaches one of those points in its yearly journey
along the ecliptic, it’s called an equinox. They always fall on about March 21 and September 23. These are the vernal
(spring) equinox and the autumnal (fall) equinox, respectively.
A representation of the ecliptic
and the apparent path of the sun
The solstices are also related to the ecliptic. The summer solstice marks the date that the Sun appears highest in
sky at noon, and this day has the longest amount of daylight for the year. The winter solstice is when the Sun appears
lowest in the sky at noon, and has the least amount of daylight for the year. These are the Sun’s high and low points
on the ecliptic.
Astronaut in zero-G?
Image courtesy: NASA
When you see pictures of astronauts floating in space, we are talking about a zero-G environment. But it is a mistake to think that there is
no gravity up where the Space Shuttle floats. Unfortunately, this misconception is widespread; we even have found it in schoolbooks.
Gravity is everywhere. Of course we know gravity from our daily lives, walking on the surface of the earth. But the pull of our planet goes
much further than that. Ever wondered why the Moon does not just 'floats away'? Exactly: the Moon and the Earth are gravitationally bound. Not
only does the gravity of the Earth reach the Moon, the reverse is true as well. Actually you know this, because the ocean’s tides are a
direct result of the pull from the Moon. And there's more. As the Earth revolves around the Sun, the gravity
of the Sun also has an affect on the tides. Without the Sun we would not have spring-tides, for example.
So how far do you have to travel to have no gravity? Well, Pluto is also connected by gravity to the Sun, so that's not far enough. Did you know
that the Sun revolves around the center of our galaxy, the Milky Way? Indeed, the Sun (actually the whole Solar System) is gravitationally bound to the Milky
Way. Going urther out, gravity still exists, and plays a major role. The Milky Way and Andromeda Galaxies are on a collision course, all because
of the gravitational pull between the two! Don't worry though - this collision will not take place for another three billion years.
So, galaxies revolve and interact with each other because of gravity. On even the largest scales in the universe, whole galaxy clusters
work together combining their gravity. Gravity is everywhere.
Now, you might ask, if gravity is everywhere how is it possible for astronauts to float? Shouldn't they fall? Well, stay tuned, because that will
be the subject of next month's AstroNews.
New Horizons Flyby of Jupiter
At the end of this month, the New Horizons spacecraft, whose destination is Pluto,
will fly past Jupiter on its way to the outer reaches of the Solar System. Launched
by NASA on January 19, 2006, New Horizons is currently traveling at 43,000 miles
per hour. After the Jupiter flyby, using its gravity to slingshot off toward Pluto,
it will be traveling at around 52,000 miles per hour.
Artist's Impression of New Horizons spacecraft Jupiter flyby
Image courtesy: Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute
This puts New Horizons at Jupiter in just over 13 months. Let’s put that in perspective
with how humans usual travel. If New Horizons were going at the speed of a car on the
highway, so around 70 miles per hour, it would take over 673 years for it to reach Jupiter
at that speed. 13 months is the quickest of any man-made spacecraft to reach the giant
New Horizons will test most of its onboard equipment during the flyby. Even though it
will be 1.4 million miles from Jupiter, it will be close enough to study the planet and
its moons in detail. New Horizons is designed to gather months of data before transmitting
it back to Earth. It will conduct tests similar to its Pluto encounter while visiting
Jupiter, so that any necessary adjustments can be made now.
After reaching Pluto in July 2015, it will spend five months studying the dwarf planet
and its three moons. It will study the geology, structure and atmospheric conditions of
those bodies. If all goes well, it will then conduct similar studies on smaller objects
in the Kuiper Belt, the region far beyond Neptune’s orbit populated with rocky, icy objects.
Visit the New Horizons website for many
more fun details about this mission, and even track where the spacecraft currently is at.
Have you ever wondered how far can you see? Of course, you can see buildings or mountains in the distance, but when you look
up in the sky you realize that you can see a lot further then you probably thought. The closest star other then the Sun, Proxima
Centauri, is about four light years away. Wow, that's about 23,498,359,696,815 or more than 23 trillion miles away.
But of course all other stars are further away then that, because Proxima Centauri is the closest star. The farthest star
visible with the naked eye is in the Southern hemisphere and is called 's Carinae'. This star is so bright, we can see it from a
distance of more then 11 thousand light years, or more then 64 quadrillion miles.
So, is that the farthest we can see? Well, no. Besides stars you can actually see another galaxy without the aid of a telescope
or binoculars, and that is the andromeda galaxy. This galaxy consists out of about 200 billion stars and is generally accepted
to be the farthest object visible with the unaided eye. The Andromeda galaxy is about 2.9 million light years away. So,
from now on, if anyone asks you if you know how far you can see, you will know what to answer: 2.9 million light years!
M31, the Andromeda Galaxy
Image courtesey NASA
So, what can we expect in 2007? If you have been a regular reader of the Astro News pages, you will understand that
every month there is something interesting happening in the sky. But a couple of events do stand out for 2007.
In March and in August of this year, we will be able to see a total Lunar Eclipse. A Lunar Eclipse is not as
spectacular (and less rare) as a Solar Eclipse, but they are still very interesting to observe. They make also perfect
science projects! Try observing the Moon over the span of 2 weeks, and see how the phases of the Moon change each day.
then as a spectacular end to the project you get to observe the total Lunar Eclipse. Unfortunately, from the US there will
be no Solar Eclipse visible, not even a partial one.
Composite Image of a Lunar Eclipse
Image courtesey epod.usra.edu
There is also good news for planetary observers: Mars, Jupiter and Saturn will all reach opposition in 2007, and will
be perfect to observe. Saturn will be the first in February, then Jupiter in June and Mars in December. Of course we will
write in detail about these events, so stay tuned.
But of course there is much more to look forward to. There will be the regular meteor showers, Shuttle launches, and chances
to see the International Space Station. And who knows... maybe even some announcements of astronomical size about a
planetarium for Austin!