Your local sky

Which way am I facing?

The lab for this module. Find true north using the sky and arithmetic, no compass, no phone.

Time: 20 minutes, needs sunshine or a visible Moon.

The question

You are standing outside. Which direction are you facing?

Your phone's compass will answer instantly, and it will be a few degrees wrong, and you will not know why. This lab answers it from first principles and tells you your own error bar.

Predict

Before doing anything:

  1. Point at where you think north is. Note a landmark in that direction.
  2. How confident are you, in degrees?

Method 1: the Sun's azimuth

Ask Moonfield where the Sun is:

moonfield sun
Sun -- Brighton (50.8225N, 0.1372W)
2026-08-16 13:22 BST

  Altitude   52.4 deg above the horizon
  Azimuth   183.7 deg  (S)

Azimuth is measured clockwise from true north: 0° = N, 90° = E, 180° = S, 270° = W.

So: face the Sun. You are facing 183.7°. To face north, turn 183.7° to your left, or equivalently, north is 176.3° clockwise from the Sun, which is very nearly behind you.

Never look directly at the Sun. Use its shadow: a vertical stick's shadow points away from the Sun, i.e. at azimuth 183.7 − 180 = 3.7°.

Method 2: the shadow-tip method

No software at all.

  1. Push a straight stick vertically into level ground
  2. Mark the shadow tip. Wait 15 minutes. Mark it again.
  3. The line from the first mark to the second points roughly east

This works because the Sun moves westward, so the shadow sweeps eastward. It is crude (good to maybe 10°), but it needs nothing but a stick, and it works anywhere on Earth including places where remembering "the Sun is due south at noon" would get you badly lost.

Method 3: local solar noon

The Sun is exactly on your meridian at local solar noon, due south in the northern hemisphere, due north in the southern.

moonfield sun --explain

This prints your local solar noon, which is not 12:00 clock time. The difference comes from your longitude within your timezone plus the equation of time, both explained in module 01.

At that instant, the shadow of a vertical stick lies exactly along your north-south line. This is the most accurate of the three.

Method 4: at night

moonfield frame --facing 90

Give it a bearing, or a compass point like NE, and it lists what is in that direction. Work backwards: find a bright object, ask Moonfield its azimuth, and you have your reference.

Check your prediction

Compare true north against the landmark you picked at the start. How far off were you?

Then compare against your phone's compass. It will probably disagree by several degrees, because:

ranges from near zero to over 20° depending where you are, and it drifts by a fraction of a degree per year.

Your stick is more trustworthy than your phone, and now you know why.

Explain

What just happened: you converted a position in a global coordinate system (the Sun's celestial coordinates) into a local one (altitude and azimuth from your exact spot), using nothing but your latitude, longitude and the time.

That transformation is the whole content of observer.py. Everything else in that file is refinement.

Checkpoint

Try it yourself

  1. Do all four methods and compare. Which was best?
  2. Mark your north-south line permanently and check it in six months
  3. Look up your magnetic declination and predict your compass's error
  4. Find true north from the Moon, at night
  5. In the southern hemisphere: check the Sun really is due north at solar noon

Questions to think about

Sun alone? (Yes, see module 06.)

where "the Sun is due south at noon" is sometimes false?

Go deeper

Next: Altitude and azimuth.