The question was:
From which hemisphere was this photo taken? To answer the question, you do not need to recognize the specific desert photographed. You just have to analyze one of the details in the picture.

Solution:

Mare Crisium on the moon is always between the 12 and 6 o'clock position when the moon is viewed from the Northern Hemisphere. But here it is in the 8 o'clock position, so it must be a Southern Hemisphere photo. In fact it was taken from the Namib Desert in Namibia, Africa at a latitude of 24 ^oS. They are seeing the moon inverted because they are "at the bottom of the world", from our perspective. For moon pictures taken from various parts of the world, click here.

The question was:

Fermi problems are estimation problems based on reasonable assumptions. At first a Fermi problem seems to be a piece of trivia unrelated to logic, but with a little thinking and basic knowledge, an estimate within the right order of magnitude is easily attainable.

Here's an example: How many high school chemistry teachers work in Canada?

Solution:

First we need to estimate the number of high school students in Canada. In most provinces children are in high school from the ages of 12 to 18, for a total of six years. The average life expectancy is close to 80 years and Canada's population is about 33 million. Because of demographics, the age-distribution is skewed away from the adolescent years, but roughly there are about (6/80)(32 X10⁶) = 2.4 million Canadians in high school.*

There are roughly 18 students for every high school teacher, and if I use the high schools I know as examples there are either 1 or 2 chemistry teachers (call it 1.5)for every 80 high school teachers.

So there are 2.4 million students(1 teacher /18 students) (1.5 chem teachers /80 teachers) = 2500. Given that a few of our estimates involved only one significant figure, we would have to conclude that there are somewhere between 2 000 and 3 000 chemistry teachers working in Canada.

*How good is this type of reasoning? Well, according to Census Canada, there were 31,612,895 Canadians aged between 0 and 80 in 2006. Stats Canada (at http://www12.statcan.ca/english/census06/data/highlights/agesex/pages/Page.cfm?Lang=E&Geo=PR&Code=01&Table=1&Data=Count&Sex=1&StartRec=1&Sort=2&Display=Page) ) does not give the actual number of children aged between 12 and 18, but they do have the "0 to 14" figure. According to the way we came up with our estimate we would guess that there should be about (14/80)( 31,612,895) = 5 532 257 children in that age group. The actual number is 5,579,835, a difference of less than 1%!

The question was:

There are 21 parts of oxygen per 100 parts of atmosphere, but only 377* parts of carbon dioxide per million.
In 1850 there were only 286 ppm of CO₂(ice core data). Let's just say that on average approximately 1.5 moles of oxygen are consumed for every mole of carbon dioxide introduced.

What percent of oxygen have we removed from the atmosphere since 1850?

*(2004 measurement = http://cdiac.ornl.gov/ftp/trends/co2/maunaloa.co2)
** Expectedly, we continue to eat up a small percentage of oxygen. CSIRO scientists have measured a small decline in oxygen between 1980 and 2000, the longest period over which such an assessment has been made.

Solution:

Proportionally for every 10⁶ molecules of air there are 210 000 parts of oxygen. In 1850 there were only 286 ppm of CO₂ ; now there are 377.
This represents an extra 91 molecules of CO₂ for every 10⁶ in the atmosphere. Stoichimetrically, 1.5 * 91 = 136 molecules of oxygen are consumed for every million. 136/210 000 * 100% = 0.065% loss

The question was:

Do you know the answer to this poetic couplet?

Why is it that ice is more dense than snow
If both are solid forms of H₂O?

Solution:

Ice's density varies from 0.850 g/cm³ to 0.920 g/cm³. Fresh snow's density varies from only 0.050 g/cm³ to 0.150 g/cm³. Snow with dendrites allows more air to fit in between crystals, lowering the density. If snow flakes are in the shape of simple needles or column-like structures, less air fits in between. But in general, compared to ice, snow contains a greater amount of air , which is why an equal volume of ice is heavier.
In between snow and ice are névé and firn. Névé is partially melted, refrozen and compacted snow. If it survives another season, its density increases from 0.50 g/cm³ to over 0.55 g/cm³. At that point it becomes firn, which, with pressure, will become glacial ice. Then you have a glacier.

References:

http://www.physicalgeography.net/fundamentals/10ae.html

http://snowcrystals.com

The question was:

At the end of the earth's Paleozoic Era, about a quarter of a billion years(2.48 X10⁸) ago, 90% of life on earth was wiped out. Yet the era left us with more than just fossils. Most of the world's coal and limestone(from which we get lime to make cement) was produced in the Paleozoic Era.
If you reduce the entire earth's history to a calendar year, with January 1st, 12 AM, being the formation of the earth, on what date does the Paleozoic Era End?

Solution:

The earth is 4.55 billion years old, and the cataclysmic end of the Era occurred 0.248 billion years ago. If we express that as a fraction of the whole, we get (0.248 X10⁹/4.55 X10⁹)*365= 19.9 days ago = Dec 12th, (0.1*24=2.4 hours),2:24 AM. Keep in mind that on the Earth calendar our present time is considered to be Dec 31, 12 PM, on the verge of beginning day # 366.
see Calendar Earth