• $1$ nautical mile (nm) $=$ the distance along a great circle subtending $1'$ (one minute of arc) at the centre of the Earth. $\ 1\ \text{nm} = 1.853\ \text{km}$ (about $1.85$ km).
• Radius of the Earth $R \approx 6370\ \text{km}$ (use the value given in the question).

Distance along a meridian or the Equator

$$\text{Distance (nm)} = 60\times\theta^{\circ}\ \ (\theta = \text{angle between the two points in degrees})$$ $$\text{Distance (km)} = \frac{\theta}{360}\times 2\pi R$$

Distance along a parallel of latitude $\alpha$

$$\text{Distance (nm)} = 60\times\Delta\lambda\times\cos\alpha \qquad \text{Distance (km)} = \frac{\Delta\lambda}{360}\times 2\pi R\cos\alpha$$ where $\Delta\lambda$ is the difference in longitude. The radius of the circle of latitude $\alpha$ is $R\cos\alpha$.

Time difference

$$\text{Time difference} = \Delta\lambda \times 4\ \text{minutes} \qquad (15^\circ \text{ of longitude} = 1\text{ hour})$$ Places to the east are ahead (add the difference); places to the west are behind (subtract).

Speed

$$\text{Speed (knots)} = \frac{\text{distance in nautical miles}}{\text{time in hours}}$$ ($1$ knot $=$ $1$ nm per hour.)