FM's Archaeo-Astronomy Topic - the basics

[Forum Monk]

Let us consider an observer, north of the equator ( I have a northern bias since it's where I live - apologies to my southern friends). Let us say as we are observing a particular star and tracking its position. We can only observe a star under two principle conditions:
1. It must be dark (doh - seems obvious, eh?)
2. Its celestial latitude must be within 90 degrees of our latitude on earth or it will be below the horizon.

In the image below, the observer is shown on the surface of the earth and his horizon is extended as a tangent line perpendicular to the observer's latitude. The earth is experiencing summer in the north and illustrated to the right of the sun (which is not seen). Notice Star C is unobservable as it is below the horizon.


Six months later the earth is on the other side of the sun, which is now off image to the right. Only Star A is observered. Star A would be considered Circumpolar as it is observable any time of the year and throughout the night it never appears below the horizon.
(Note - which stars are observed as circumpolar is very much dependent upon the observer's latitude.)


Precession has a dramatic effect, given enough time. Consider the next image after 12,900 years when precession has completed 1/2 of a cycle. As in the first image, the north is experiencing summer only now when the earth is to the left of the sun. Star A and B are visible but at significantly different positions above the observer's horizon.


Six months later, it is winter in north, with the earth to the right of the sun. Star A is no longer circumpolar, rather Star B has become visible all year around and now Star C is visible.


Here is a summary of the observations:

Before precession -
Star A is observered all year.
Star B is observed only summer.
Star C is never seen.

After precession -
Star A is observed only in summer.
Star B is observered all year
Star C is observered in winter

[Beagle]

Great thread you've got going FM! I don't have any questions so far, but I just wanted to toss an attaboy to you.

[Forum Monk]

In the previous post it should be possible to see how precession changes the local horizon signficantly over the passing years. This has a definate affect on the apparent location of stars. But, the sun, being very close (in astronomical measure) to the earth, is not affected in any way. It should now be somewhat clear, that like the orbit of the earth around the sun, precession shifts the seasons, but the axial tilt of the earth never changes; only the direction of the polar axis. In terms of astronomical observing the sun is much more consistent and its movement pattern is quickly understood. Within in one year of time it will sweep out a full 47 degree change in position moving from due east to 23.5 degrees north of due east in northern summer to 23.5 south of due east in northern winter and back to due east. This movement provides more hours of sun light in the summer and less in the winter, since the sun it not rising as far above the horizon before it sets again.

Ancient astronomers within one year could build a simple solar observatory by marking an observing location then while standing at that location, mark the sun's position on the horizon each morning. Very soon the astronomer will realize the sun is always moving either northward or southward and back again. Now the astronmer can denote the extreme northern rising position and the extreme sourthern rising position and so with three simple markers have an effective observatory for denoting the passage of time and seasons.

But...

The distance between those two markers denoting the sun's extremes will vary as a function of the observer's latitude on earth. The farther from the equator, is the observer, the wider are the markers because the angle of the ecliptic with respect to the local horizon.

In the picture below,an ancient astronomer has marked the extreme rising points of the sun from his vantage point located directly on the earth's equator. The obelisks are 23.4 degrees from the center point.



Meanwhile, another astronomer at 45 degrees north latitude has done the same, but because the ecliptic tracks are skewed, the obelisks are a little over 33 degrees apart.

[War Arrow]

Great thread you've got going FM! I don't have any questions so far, but I just wanted to toss an attaboy to you.

Likewise.

[Forum Monk]

The Moon
The moon orbits the earth at an approximate average radius of 384,000km and takes 27.3 days to make a complete revolution around the earth. The moon also rotates upon its own axis, but because its rotation is synchronized with its orbit, we pretty much always see the same face. In other words, the lunar year and lunar day both take about 27 earth days. This leaves once side of the moon always facing away from earth, the so-called dark side of the moon.

The side we see, is not always fully lit. This passing through phases of light and dark is caused by the geometry of the positions of the earth, moon and sun. This geometry causes the moon to appear to pass through a complete cycle of darkness and light every 29.5 days. Obviously, as the moon orbits the earth it will periodically pass between the earth and the sun. During these times the moon appears dark, as the side facing away from earth is receiving the sun light, while the side face the earth is in shadow. Then some 14-15 days later, the opposite condition occurs and the side facing earth is fully immersed in light as illustrated in the image below.



As can be seen in the image, the orbit of the moon is inclined about 5 degrees. This fact, creates additional dynamics with respect to the orientation of the moon and the frequency and location of eclipses, but more on that later.

[Forum Monk]

The five degree inclination means that depending on where the moon happens to be in its orbit, it can appear as much as 5 degrees above or below the ecliptic. (Remember, the ecliptic is the imaginary line the sun appears to follow throughout the solar year.) Given this fact, there will be times when the moon will appear to rise five degrees farther north or south from the extreme northern and southern rising points of the sun (which occur on the summer and winter solstice dates).



So at some of the higher northern or southern latitudes, this five degrees will appear much more extreme.



This image above, for example depicting the extreme northern rising point of the moon from a latitude of about 45 degrees north. There would be a corresponding extrema to the south.

Since the moon orbits the earth every 27.3 days, very rarely, will the moon actually appear at the extremes as the correspondence of the solstice and the moon being at its maximum or minimum is very rare. These are known as major lunar standstills (the moon appears to stop or standstill in it northerly or southerly travel along the horizon), and occur about every 18.6 years.

See this article for more explanation:
http://en.wikipedia.org/wiki/Lunar_standstill

[Forum Monk]

In the first post about the moon, I wrote of the phases of moon, but now realize that by saying one side of the moon is illuminated and the other in shadow, I am using bad terminology. Saying the unlit portion is in shadow is not technically correct and may cause some to visualize the earth casting a shadow over the moon. That was not the intent of the expression. Therefore it is better to say the phases of the moon are the result of angle of the sun light falling upon the visible surface of the moon. At various times the surface is partially lit and partially unlit (not in shadow).

Sorry if I caused any confusion.

[Forum Monk]

The point at which a body crosses a plane is known as a node. For example, when the apparent motion of the sun appears to cross the celestial equator, or the moon appears to cross the ecliptic, these are called nodes. If the body is crossing the plane from south to north, the node is an ascending node and if crossing from north to south, a descending node.

Considering the moon. An eclipse can only occur when the moon is directly between the sun and earth (a solar eclipse may occur) or with the earth directly between the sun and the moon (a lunar eclipse may occur). Therefore, an eclipse can only occur when the moon is either new or full and at the time the moon is crossing one of its nodes. (Lunar nodes are with respect to the ecliptic.)



You should be able to see, that as the earth and moon revolve around the sun, the sun is not always in line with earth and moon when the moon crosses a node. Often the moon's shadow or the earth's shadow passes to the north or south and does not cross the opposing body. For this reason, an eclipse does not occur everytime the moon orbits the earth. In fact, for a solar eclipse to be visible from a given location on the earth, the correct combination of positions required for the moon's shadow to pass over a given location on earth is relatively rare. But the ancient Babylonians did make an important discovery: the correct combination will reoccur on a periodic basis. The cycle repeats every 18 years, 11 days and 8 hours. This was later called the Saros Cycle. But since there is a fractional number of days in the Saros cycle (8 hours = 1/3 day), the eclipse path will shift westward, one-third of the earth's diameter, each Saros Cycle. Therefore after 3 cycles (54+ years) the eclipse will occur in, more or less, exactly the same location as before.

[CShark]

As someone said earlier, Keep 'em coming!

FWIW, I used to be an avid amateur astronomer: ground my own 8inch newtonian, spent many a frozen winter's night lying in the snow with a pair of binocs, trying to learn where things were. Spent my geeky youth pouring over star charts and re-reading 'Brief History of Time'. Love the posts, there is much I did not know and likely more to come.

I'm going to be reading your posts concerninig Stonehenge (which I am assuming you are headed to eventually) with great interest.

Keep em coming!

Bob

[Digit]

Nah Nah! WA. The earlier Springs are all down to global warming mate!
I hate to tell you FM but you're beating your head against the proverbial. On a quiz show this evening the question was, which planet of the Solar System orbits the Sun once every 365 and a quarter days?
Reply. Neptune! I kid you not!

[Forum Monk]

There comes that special night when the moon makes its first appearance in the evening sky after passing through the "new phase" hidden in the bright light of the sun. It appears as a thin cresent low in the western sky, immediately after sunset. To many of the ancients, this appearance marked the first of many nights in which the moon would eventually rise to prominence in the night-time sky, reach fullness, then eventually fade into the brightness of the dawn sun. Obviously the ancients took note and discovered that from one first appearance to the next there were about 30 sunsets. Thirty days, for the moon to march across the sky from new to newness. (Anyone care to guess the root word of the germano-english word "month"? If you guess moonth it's incorrect but you have the idea.) If I were an ancient astronomer marking new moons with respect to sunsets (days), this would be my observation log:
Cycle Days
.1......30
.2......29
.3......30
.4......29
.5......30
.6......29
.7......30
.8......29
.9......30
10......29
11......30
12......29
13......30
Yep, I see a pattern. Now I can run and tell the king, sire, I can predict the next new moon and he would be suitably impressed. Now in order to maintain my favorable status, I decide to count off the number of days between northern solar stand-stills (the summer solstice). Next summer I find I have made 365 tick marks, one for each day. Now with some basic counting, I can see that within the 365 period, I can mark-off 12 lunar cycles (the alternating 30/29 days) but must wait another 11 days for the solar cycle to complete. So now I tell the king, we can track a year by counting 12 luna's and 11 days. And the king says, 11 extra days..hmmm..I'll take the 12 luna's and we'll give the 11 days to the gods, in the form of feast days. And so we have just devised the first luni-solar calendar.

Is this what the ancients did? Well perhaps. We know that some societies did not use the alternating day method. Instead they settled on 12 luna's ..er.. months of 30 days with 5 intercalary days to close the year. Some, made no attempt whatsoever, to reconcile the lunar months with the solar year. Such systems would allow their "new year" to slip through the solar year eventually realigning when sufficient extra days would accumulate amounting to a 13th month.

I am of the personal opinion, that nearly all ancient calendars were based one way or another on the movements of the moon (whether aligned with solar year or not). The moon was a very important object as the brightest object in the night-time sky and the fastest moving. It is said that 'moon' may be from the proto Indo-European word me^ meaning "measure" as in time. There is much more to the cycles of the moon which support that conclusion. But this is enough for now.

btw
What if the king one day said to me, look luna's are great for dividing the year but this intercalary stuff is driving me nuts. Take the extra days and put them into the lunas. So I surmise..if I take six of the extra days I can give one each to the 29 day lunas, giving 12 lunas of thirty days. Then I can scatter the reamining five days according to the king's wishes, so:
Cycle Days
.1......30 + 1 extra day for Jupiter
.2......30
.3......30 + 1 for Mars
.4......30
.5......30
.6......30
.7......30 + 1 for Saturn
.8......30
.9......30 + 1 for Mercury
10......30
11......30
12......30
13......30 + 1 for Venus

If a future king favors another god, he may rearrange as it pleases him.

hmmm - starting to look like a modern calendar.

[Forum Monk]

...spent many a frozen winter's night lying in the snow with a pair of binocs, trying to learn where things were.

This is the kind of madness that consumes amateur astronomers. You brought back memories. I am sure our families thought we were nuts as only a few really understand the allure.

Welcome to the forum.

[Digit]

I was about 10 when a neighbour showed me a very unsteady image of Jupiter in a tiny 'scope, I was hooked from that day FM.

[Forum Monk]

There's another kind of madness Digit. Many amateur's will give up sleep remaining outside until the wee-hours just to take advantage of favorable viewing conditions. Especially in summer when nights are short or when clear skies are rare. So much to see and so little time to see it.

And as for allure. When looking at the Andromeda galaxy, its not only the fact you looking at the sister galaxy to our milkyway. You are looking across 2.5 million years of time; when perhaps an ancient hominid was gazing upward at similar sights.

Astronomy is cool

[Digit]

It tends to cut you right down to size Monk, you realise just how insignificant we really are.