Astronomy · Rung 12 of 12

Observing for real

A good night is planned in daylight. You check the Moon's phase (full is the worst for everything except the Moon), the sky forecast, and what is high at the hour you will be out. You give your eyes twenty minutes in the dark, no phone, and use red light. You learn averted vision, because the sensitive part of the retina is not at the centre. And from Sweden there are two gifts and one sentence: the aurora when the Sun misbehaves, the pitch-dark January sky, and a summer where night simply never falls.

Where this sits

Comes after
Instruments

Exercises from this rung

Generated by the app, from easiest to hardest

  1. To see a very faint object it helps not to look straight at it. Why?

    • The rods are not at the centre
    • The atmosphere bends the light
    • The eye tires of staring
    • The object is moving

    Why Averted vision. The centre of the retina, the fovea, is all cones: excellent for daytime detail and colour, terrible in low light. The rods, a hundred times more sensitive, sit around it. Look about 15° to one side of the object and its light falls on them, and it appears. The trick genuinely works on Andromeda or a nebula.

    easier

  2. In June, around midnight, very high silvery clouds sometimes appear. What are they?

    • Ordinary clouds lit by the city
    • Faint auroras
    • Meteor dust
    • Noctilucent clouds, 80 km up

    Why Noctilucent clouds: ice crystals in the mesosphere, some 80 km up, so high that they still catch sunlight when it is night below. They only appear in summer and between 50° and 65° of latitude, so Sweden sits right in the good band. They are the summer's consolation, when there is no night for anything else.

    in between

  3. What produces an aurora?

    • Solar particles that excite the air
    • Cometary dust burning up
    • Reflection off the polar ice
    • Lightning in the upper atmosphere

    Why The solar wind brings charged particles; Earth's magnetic field funnels them to the poles, where they hit the atmosphere 100-300 km up. Excited oxygen emits green at 557 nm and red higher up; nitrogen gives violet and pink. Hence the auroral oval around each magnetic pole, and hence northern Sweden being one of the best places on Earth to see them.

    harder