We can look everything and anything up on our devices with relative ease, but sometimes just knowing a rough number is useful for a quick calculation in your head. Here's a list of numbers that have stuck around in my head, and some more that I'd like to get stuck in my head.

Symbols

• r_thing = Radius of thing

Speed & Time

• 1 m/s is about 2 mph (2.2 mph)
• Speed of light ≈ 3 × 10⁸ m/s (300,000 km/s)
• Light takes 8 minutes to get from Sun to Earth

Temperature

• 10ºC = 50ºF
• 40ºC ≈ 100ºF

Distance

• Circumference of Earth is 40,000 km
• Moon is 400,000 km from Earth (≈ 1 light-second)
• r_moon ≈ 1000 miles
• r_mars ≈ 2 × r_moon
• r_earth ≈ 2 × r_mars
• r_sun ≈ 100 × r_earth