Stage 1

The sextant

It measures one angle: between two things you see at once. Here, a star and its reflection.

Starlight (gold) bounces off the index mirror, then the horizon mirror, into the telescope. Through the clear half you see the horizon or the tray. Moving the arm swings the star until both images meet.

Read the vernier

Numbers run right-to-left, as on your arc. Read the arc at the vernier's zero, then add the vernier line that lines up exactly. Your vernier is probably finer (10″ or 20″ steps). The method is the same.

Index error, every night

Set near 0°, look at one star, turn the tangent screw until its two images merge. The reading is the index error. On the arc: subtract it. Off the arc: add it.

Measure it outside, after 20 minutes in the cold. Cold metal shrinks and the error can change. Do it three times and average.

Cold-weather care

Condensation

Never carry the cold sextant into a warm room unbagged. Seal the case in a plastic bag and let it warm up for an hour.

Breath

Breath freezes on mirrors and the eyepiece. Breathe out sideways, and wear a buff.

Gloves

Thin liner gloves under mitts, so you can feel the tangent screw. Bare fingers stick to brass.

Stiff screw

Old grease thickens. Turn gently, never force it, and don't re-oil it blind.