Latitude and longitude, the major surface reference points (equator, tropics, polar circles, Prime Meridian and poles), and the effects of the atmosphere on astronomical observations.

What this dot point is asking

Edexcel statements 1.4 to 1.6 want you to use the latitude and longitude coordinate system, to use the major divisions of the Earth's surface (equator, tropics, polar circles, Prime Meridian and poles) as astronomical reference points, and to understand how the atmosphere affects observations through sky colour, skyglow and twinkling.

Latitude and longitude

Together, a latitude and a longitude pin down any point on the Earth's surface, just as two coordinates locate a point on a grid. Latitude matters in astronomy because it sets which stars are visible and how high the celestial pole sits above your horizon (the altitude of the pole equals your latitude). Longitude matters for time, because the local solar time depends on how far east or west you are, which links forward to time zones and the determination of longitude.

Reference points on the Earth's surface

These circles are set by the Earth's $23.5$ degree axial tilt. The tropics mark the most northerly and southerly latitudes where the Sun can be directly overhead; the polar circles mark where the Sun can stay below or above the horizon for a full day (polar night and midnight Sun). They are used as astronomical references because they predict where and when the Sun and stars appear for a given observer, which is essential for planning observations.

How the atmosphere affects observations

All three effects are reasons astronomers prefer dark, high and stable sites, or put telescopes in space. Skyglow is worst near towns and cities; twinkling is worst when the air is turbulent and is reduced at high altitude where there is less air to look through. Planets twinkle less than stars because they show a small disc rather than a point, so the shimmering averages out. These atmospheric limits return in Topic 6 (observing conditions) and Topic 13 (why telescopes go above the atmosphere).

How Edexcel examines this

This is naked-eye Paper 1 content that supports both definition and applied marks. Coordinate questions reward defining latitude (from the equator, north or south) and longitude (from the Prime Meridian, east or west) and may ask you to read or plot a position, or to state the latitude of a named circle. The reference points are tested by recall and by linking them to the Sun's behaviour (the tropics and the overhead Sun, the polar circles and the midnight Sun). The atmosphere questions reward precise distinctions: skyglow is scattered artificial light reducing contrast, twinkling (seeing) is turbulence blurring images, and blue sky comes from scattering. Examiners reward the synoptic point that these effects drive the choice of dark, high observing sites and the use of space telescopes, so connect them forward. A common error is to swap the two coordinates or the two atmospheric effects, so define each carefully.

Try this

Q1. State what latitude measures and where it is $0$ degrees. [1 mark]

• Cue. The angle north or south of the equator; $0$ degrees at the equator.

Q2. State what causes the twinkling (seeing) of stars. [1 mark]

• Cue. Turbulent, moving layers of air in the atmosphere bending the starlight.