Astronomy in a nutshell

started on 05.09.1998 - updated on 05.09.1998

Intro
Structure of the sky
Ecliptic
Earth
Moon
Planets
Xephem

Intro

Sometimes, in clear nights, I love to walk out and watch the sky, spangled with stars.
To love something and to know about something seem quite familiar to me, so perhaps information about the sky, sun, moon, planets and stars will make my readers know about that more, and perhaps love it more.

I think it's nice to know sometimes that anything out there can do well without any human noise, spreaded over a dust bit called earth.

Structure of Sky

We live on Earth. It is a planet that rotates around its axis and moves around the sun. Every move takes one year.
Some other planets do move around the same sun. Germans have a sentence to remember their names, from near the sun to far:

Merkur (Mercury)	Venus	Erde (earth)	Mars	Jupiter	Saturn	Uranus	Neptun	Pluto
Mein	Vater	erklärte	mir	jeden	Sonntag	unsere	neuen	Pläne.
My	father	explained	me	every	Sunday	our	new	plans.

(For those who want to know what this sentence means - actually nothing.)

There are many more Stars, nebulae and galaxies in the universe. We call them stars.
They move, too. But they are so far away that in a man's life their movement cannot be seen, so they SEEM to be pinned on night's black ceiling.

Ecliptic

While Earth moves in about a circle around the sun (which stands in the center), astronoms use to put a plane into this circle, called ecliptic.
This plane touches sky's sphere, and so can be drawn into star maps.
The other planets move nearly in the same plane, with only slight aberrations. So if we can see them, they must always be near the ecliptic.
On our viewable sky, the ecliptic is always the way that Sun walks.

Earth

Grab an apple now, and rotate it around its axis with light from your desktop lamp on it. You should also pin a pitch in there, just where you are.
On my apple, you see its axis, the equator (with an E on it), and the sunlight coming from the left. I also put a pinch on it, as I live somewhere on the northern hemisphere.
Obviously, there is summer on the northern hemisphere, at the moment, and evening dawn starts in my city.

Usually, I see Sun raise in the east, move to a place somewhere in the south, and set in the west. This means, it moves from the left to the right.
Now pull the pinch, and put it onto the apple's equator. And see what happens: The sun rises in the East, moves to the top of the seeable sky, and sets in the west.
Somewhere on the southern hemisphere, Sun rises in east, moves to a place in the north, and sets in the west.
On a pole, sun is near the horizon all the year (half a year over it, half a year under it), always moving around you.

Moon

Earth has a satellite, which moves around Earth in approx. 30 days, and its moving plane is NEARLY the ecliptic.
Grab an apple (for Earth) and an egg (for Moon) now, shine them with a desktop lamp and try to understand Moon's phases.

On its movement, Moon sometimes stands in between Sun and Earth. We (on Earth) can only face the side where Sun doesn't shine on, and call this New moon.
As you can see, we can see New moon and Sun at the same time. This means, New moon is on day's sky, and though it is even harder to see.

If Moon stands at the side, we see half its face shone on by Sun. Moon's face forms a C or the other way 'round like Þ. Sun is just raising or setting, because it stands just under the horizon of our Moon viewing earthling.
If Earth stands between Sun and Moon, we see the full face of Moon shone on: Full Moon.

A certain moon phase can only occur at a certain daytime. The following is for the northern hemisphere, any other area will come up to you with your lamp, your apple, and your pinch.

I said: We see the stars move. That's not quite right, because Earth turns, and the stars stay on their places. This means, Earth's surface moves from west to east all time.
So one half moon, the one that forms a mirrored C (like Þ), can be seen best at evening dawn.
Full moon is viewed best at midnight.
Moon shaped as a C is viewed best in the early morning. This means, not by me, for I like to sleep long.

Now we can understand why there are lunar eclipses: If Earth is EXACTLY between Sun and Moon, its shadow falls on the moon. This MUST occur on Full Moon.
It does not occur every full moon, because Moon's rotating plane is not exactly Earth's rotating plane. Neither does a solar eclipse (what means Moon blanking out Sun on certain areas of Earth) occur every New Moon.

Planets

Earth moves around Sun in one year.
The inner planets move much faster, and they always stay near the sun. Half venus can only be seen on evening or early morning, that's why Germans use to call her Evening's Star or Morning's Star sometimes.
Mercury is too small to be viewed by the eye.

The outer planets move slower than Earth, and take more time for one turn.
That means, some times in year they are near or behind the sun, on day's ceiling, and cannot be seen at all.
On other times, they appear brightly at midnight. In September 1998, Jupiter can be viewed around midnight, near the Full Moon, with Saturn coming up after midnight.

Xephem

is available for Linux. It shows for every day the planet's constellation so that you can see whether you can see a certain planet at all (that means on night's sky, on day's sky you can't).
It tells me that Jupiter and Saturn can be seen from July to November 1998, while April 1999 will produce a nice half Venus (in the morning or evening) and a recommended Mars on midnight.

This page hosted by Get your own Free Home Page