New observers often find the night sky confusing. What you see depends on where you are and the time of night you look skywards. Our observing tips will help you make sense of it all.
The view depends on the time of night and the time of year
The sky is constantly changing. It turns on a daily cycle (one complete turn every day) and on an annual cycle (one complete turn every year).
- Daily changes are caused by the spinning of the earth on its axis. The stars rise in the east, reach their maximum altitude when they cross the meridian (an imaginary line drawn through the sky from north to south), then set in the west.
- Annual changes are caused by the orbiting of the earth round the Sun. Like all seasonal changes, they happen gradually. They give us different skies in spring, summer, autumn and winter.
The view depends on where you are
Prime Sky objects and events should be visible from just about anywhere within the UK, although times and positions in the sky may not be quite the same. Prime Sky observations are, strictly speaking, accurate only for the Royal Observatory at Greenwich in London (longitude 0° [the Prime Meridian, hence Prime Sky]; latitude 51° 28' 38'' N).
- North or south of Greenwich: objects will be higher or lower in the sky
- East or west of Greenwich: event times may be earlier if you live to the east; later if you live to the west
- Within the UK: variations in time and position are usually too small to worry about; the further away you live, the more pronounced they are
- Elsewhere within the northern hemisphere: Prime Sky objects and events may still be visible, especially if you live at similar latitudes – check locally for times and positions
Nominal viewing times
Many Prime Sky objects are visible for most of the night. Others are visible only at specific times. For all-night objects, we suggest nominal viewing times of 21.00, 22.00 or 23.00 depending on the time of year.
If you look earlier, objects will be to the east of where we say; if you look later, they will be to the west. They may also be higher or lower in the sky.
How to find a Prime Sky object
Every object or event featured on Prime Sky includes two simple pointers on where to look.
tells you which direction to look
[North • NE • East • SE • South • SW • West • NW]
If you do not have a compass and you live in the northern hemisphere, you can use the stars to find north.
Find north using the stars
You can find north on any clear night in the northern hemisphere by following the stars. One of them, Polaris (also known as the Pole Star or North Star) happens to be very close to the north celestial pule, a fixed point in the sky directly above the North Pole on earth. When you face the Pole Star, you are facing north.
The first step is to find the familiar saucepan shape of the Plough. The two stars that form the side of the saucepan opposite the handle are known as the Pointers. They point outwards from the saucepan directly to Polaris. The diagram shows you how.
tells
you how high up to look (the altitude)
[0° (towards the horizon) • 15° • 30° • 45° • 60° • 75° • 90° (directly overhead)]
If you are not sure how high an object is in the sky, you can use your hand as a scale.
What’s the altitude of that star?
The altitude, or height of a star in the sky, is measured in degrees above the horizon. A star at 0° would be right on the horizon, while one at 90° would be directly overhead at the zenith, the highest point in the sky.
You can use your own hand to estimate altitude. Start by extending your right arm and hand in front of you at eye level. Keep your hand flat and vertical with the fingers close together, thumb at the top and little finger at the bottom. Now bend your hand to the left so that the palm faces you and your fingers point to the left.
At that distance, the width of your hand from top to bottom is about 10°.
You can check by using your hand to estimate the altitude of the Pole Star, which is about 51° (five hand-widths) above the horizon in southern England and 58° (six hand-widths) in northern Scotland. From the horizon to the zenith should be nine hand-widths.
Another simple trick is to divide the distance between the horizon and the zenith into thirds. The lowest third stretches from 0° to 30°; the middle third, 30° to 60°; the highest third, 60° to 90°.
The view depends on weather and visibility
The clearer the skies, the more you will see. For the best view, avoid the following:
- Bright moonlight outshines all other objects in the sky. Choose a night when there’s no moon (known as new moon) or view before midnight during the moon’s last quarter and after midnight during the first quarter.
- Light pollution from cities and other built-up areas gives the sky a dull orange glow that obscures faint objects (see Campaign for Dark Skies for more information).
- Clouds can spoil your view. Here is the BBC’s weather forecast for London tonight.
Get a better view
Here’s how to see Prime Sky objects at their best.
- Find a location that is well away from light sources.
- Give your eyes at least 15 minutes to adapt to the dark. Your dark-adapted vision will be at its best after about half an hour.
- Do not switch on lights or torches. If you need to read something in the dark, use a torch covered by a transparent red film.
- Faint objects are sometimes better seen using a technique known as averted vision. Do not look directly at the object, but slightly to the side. That’s because the area surrounding the centre of your eye is more sensitive than the centre is to low levels of light.
- Although Prime Sky objects can always be seen with the naked eye, a pair of binoculars will show you fainter objects and more detail, especially on the surface of the Moon. Binoculars are cheaper, more portable and easier to use than telescopes. Lean against a wall or other fixed object to steady your view.
- A telescope shows even fainter objects including the rings of Saturn and the four larger moons of Jupiter. Bear in mind that telescopes can be fiddly to use and that their images are upside down.
Get comfortable
The more comfortable you are, the more you will enjoy your stargazing.
- On cold nights, wrap up well. Several layers of thin clothing are better than a few layers of thick clothing. Hats and gloves are helpful too.
- For longer stargazing sessions, a comfortable reclining chair makes a huge difference. A hot drink in a flask helps too.
