7 Gpeople

While at a conference in Istanbul, I went to the İSTANBUL ARKEOLOJİ MÜZELERİ, which was an absolutely fascinating archeology museum. Istanbul has featured prominently in the growth of civilization, and I struggled to keep track of the many different civilizations and cultures that occupied the region at one time or another. I had to find a youtube video to help me sort it out.

There was a very nice exhibit on Troia (Troy), which apparently really existed; it's ruins were unearthed in a farm field not too far from here. It was settled, destroyed, and resettled in 9 different epochs before being ultimately abandoned. Even back then, it seems that anything you dug up had a 1000-year history. Buildings were built on top of buildings. The reconstruction of the different settlements was interesting.

It was really hard for me to get my head around how small cities were in comparison to now. Istanbul currently has 13 Mpeople living in its greater metropolitan area. In 3000 BC, that was the human population of the world. The largest cities in antiquity were ~250 kpeople. It made me wonder if all of our advancements in technology and culture in the last couple hundred years could be attributed strictly to 1) more people to do the work and 2) longer tails on the normal distribution of people with various abilities.

So just how many people were there as a function of time? The log-log plot above from wikipedia shows current best estimates. Apparently, some 70 kyears ago, possibly as a result of a major volcano eruption, the human population was reduced to something on the order of 1000 to 10,000 "breeding pairs." Since then, the population rapidly recovered to several million, where it remained stable until agriculture was developed. This is all in a nice video tracing genetic migration via mDNA.


Since then, there has been exponential growth (a line on log-log plots) with a transition to a slower growth coefficient at ~400 BC. Occasional Black Plagues aside, the human population has increased dramatically. I remember hearing once that half the people who ever lived are alive right now. That's actually definitely false--it's closer to 6%. Also, everyone seems to think that population growth is accelerating (remember this video from the 80's ?). The graph above definitely shows that's not true, either.

But what is true is that this growth cannot continue unchecked without hitting its head on something, be it food supply, global warming, danger of pandemics, warfare, declining birth rates, or whathaveyou. It's estimated that in October of this year, 2011, there will be 7 Gpeople on the planet. This map shows where the population currently is, but it's estimated that much of the growth in the next century will happen in poverty-stricken Africa. Things pretty much have to plateau around 10 Gpeople, though.

So what to do? The most effective ways to reduce birth rates, which is key to controlling population growth, global warming, saving the environment, and many of the rest of our problems are:

1. contraception
2. improving the standard of living (ending poverty)
   
3. education (and education about contraception)
   
4. and reducing infant/child mortality

That last item is counter-intuitive. The reason it is important is that when survival rates are low, couples have more children to compensate, including a buffer for uncertainty. Having a predictable path from birth to adulthood allows for more precision in family planning.

The "New" Zodiac and the Earth's Spin

The latest rage in popular astronomy seems to be the realization that the Sun may now move through 13 constellations instead of 12. I haven't personally checked this, so I'm going to take their word for it. What's bothering me is something else that's being repeated in connection--that this is "due to shifts in the earth's rotation and orbit", or more flagrantly, that "since the zodiac periods were established millennia ago, the moon's gravitational pull has made the Earth 'wobble' around its axis in a process called precession".
Someone (I guess, me) needs to make it clear that any shift in the apparent path of the Sun through the background constellations can only be caused by a change in the inclination of the Earth's orbit (possibly as a result of the precession of the Earth's slightly inclined orbit around the Sun), not the precession of the Earth's axis of spin.

The reason the Sun appears to move through background constellations is because we're moving around the Sun (see figure to left). From the perspective of Earth (blue dots), the Sun (orange) appears in front of a different set of stars at different times of year, when the Earth is at different positions in its orbit. The project of the Sun from the perspective of the Earth is shown with dashed lines.

Now the thing is, that dashed line from the Earth through the Sun doesn't depend at all on the Earth's spin. To see this, imagine you remove the Earth from one of those locations and instead place yourself floating in space. You could turn any which way you want--upside-down, rightside-up, twisted alley-oop--the Sun is going to be in the same place relative to the stars behind. Similarly, the Earth's spin (and any precession in that spin axis) can change the orientation of the Earth, but makes no difference in what stars the Sun appears in front of.

What actually can change where the Sun appears relative to background stars is the physical location of the Earth.
This is a change in the Earth's orbit, not spin. One way to change where the Sun appears relative to background constellations is to move the Earth to different places in its elliptical orbit. This will move the Sun through the standard set of zodiacal constellations. In order to move the Sun out of the normal zodiacal progression, you need to move the Earth up or down. The way this can happen is if the plane of the Earth's orbit precesses around another axis, so that the "high" point in the orbit moves slowly around the Sun.

So to clarify, if the Sun appears to move through a new constellation, it is because the Earth's orbit around the Sun has changed, not because the Earth's spin has changed.