Earth from space: the view from orbit
Orbit is a useful vantage point for geography questions, partly because it settles arguments about what can be seen from up there, and partly because a few places on earth are defined entirely by their relationship to it.
GPS satellites must correct for what, or positions drift by kilometres a day?
Time dilation
Clocks on satellites run faster than clocks on the ground by about 38 microseconds a day once both special and general relativity are accounted for. Uncorrected, that is roughly 10 km of error a day.
Source: US Naval Observatory GPS timing documentation
Why are rocket launch sites often built near the equator?
The Earth's spin helps
The surface moves about 1,670 km/h at the equator against nearly zero at the poles, and that speed is free velocity towards orbit. Kourou in French Guiana sits at about 5 degrees north for this reason.
Source: European Space Agency, Guiana Space Centre
How long does the International Space Station take to circle the Earth?
About 90 minutes
At roughly 400 km altitude and 28,000 km/h, one orbit takes about 90 minutes, so the crew sees around sixteen sunrises a day.
Source: NASA ISS orbital parameters
How high must a satellite orbit to stay above the same spot on the equator?
About 36,000 km
At about 35,786 km the orbital period matches one day, so the satellite appears fixed. This is why satellite dishes point at a fixed spot in the sky.
Source: Standard orbital mechanics; ITU geostationary arc
Can national borders be seen from space?
A few can
The India-Pakistan border is floodlit and visible at night, and the Haiti-Dominican Republic line shows as a sharp deforestation edge. Most borders are invisible.
Source: NASA Earth Observatory imagery
What is striking about satellite night imagery of the Korean peninsula?
North Korea is almost entirely dark
South Korea blazes with light while North Korea is dark except for Pyongyang, making the border visible from orbit at night as a hard line.
Source: NASA Earth Observatory Suomi NPP night lights
What shape is the Earth, precisely?
An oblate spheroid
Rotation makes the equatorial diameter about 43 km greater than the polar one, a difference of roughly 0.3 percent, which is why the planet looks like a sphere in photographs.
Source: WGS84 reference ellipsoid
What is the Moon doing relative to the Earth?
Drifting away by 3.8 cm a year
Tidal interaction transfers angular momentum, pushing the Moon outwards and slowly lengthening Earth's day. Laser reflectors left by Apollo missions measure it directly.
Standing exactly on the North Pole, which way is east?
There is no east
All lines of longitude meet at the pole, so every direction from it points south and east and west lose their meaning. Time zones are equally meaningless there.
Source: Standard spherical geography
What causes the seasons?
The Earth's tilt
The 23.4-degree tilt changes how directly sunlight strikes each hemisphere. The Earth is actually closest to the Sun in early January, during the northern winter.
Source: Standard astronomy; Earth's orbital elements
Why is a solar day not exactly 24 hours everywhere in the year?
Earth's orbit and tilt
Earth's elliptical orbit and axial tilt together make the equation of time vary by up to about 16 minutes across the year, which is why a sundial and a clock disagree and why the earliest sunset is not on the shortest day.
Source: Standard positional astronomy