Activity 5 – Under the Milky Way

Context

Activities 1 to 4 in this strand developed the skills needed for stargazing. In this Activity you go stargazing and put those skills to work.

Specific Learning Outcomes

You will experience the night sky on a clear night, and be able to identify the brightest stars and constellations for yourself. Depending on light conditions, you will also see the Milky Way.

Planning Checklist

When you are planning a stargazing session, you should consider the following factors.

Star Wheels

Red Torches

Light Pollution

Moon

Horizon

Sunset Time and Season

Weather

Clothing

Planets and Satellites

Binoculars and Telescopes

Each of these factors is discussed in detail under its own heading, below.

Star Wheels

Activities 1 to 4 in this strand cover direction-finding as well as Star Wheel making. It is best to work through these before trying a serious stargazing session. All stargazers at a session should have their own Star Wheel and know how to use it.

Beginners should start with the City Wheel even at a dark sky location where the Milky Way is clearly visible. This is because the City Wheel is simplified and is by far the easiest way to get started using a Star Wheel. But make sure you have copies of the Milky Way Wheel available to answer the inevitable question: “What star is that?”

Red Torches

The human eye has two types of light receptors: rods and cones. The cones are specialised for colour vision while the rods are specialised for faint light. The rods cannot even detect colour, but they can see much fainter light. This is why things look black-and-white by moonlight – there is not enough light for colour vision.

The light from very bright stars is just bright enough for colour vision. That is why some bright stars look red, orange or blue. Most stars, however, are too dim for colour vision. We see them with our rods only, and they appear plain white.

Over a period of 20 minutes in the dark, the rods become more and more sensitive to dim light. This is called dark adaptation. It means that after about 20 minutes of stargazing you can usually see a lot more stars than you could when you started.

As soon as you look at a bright light this extra sensitivity goes away. It takes another 20 minutes for it to come back. This means you do not want to flash a torch around when you are stargazing, but you do want to see your Star Wheel. The solution is quite simple.

Rods have a low sensitivity to red light. You can use a dim red light without losing your dark adaptation. You can look at you Star Wheel with a dim red light, and still see the faintest stars when you look up at the sky.

Making a red torch is simple. Attach a sheet of red paper or cellophane to your torch with a rubber band. Use as many layers as needed to make the light dim. You should be only just able to read by it. One or two layers of red paper is often enough, but with red cellophane you might need four or more layers.

If you are stargazing in the city surrounded by streetlights then a red torch is not much use – the streetlights will prevent you from dark adapting anyway. But even in the city it is worth making the effort to find a stargazing place where you are shaded from streetlights and house lights. This will allow your eyes to dark adapt and you will be surprised how much more you can see. If you find such a spot you will definitely benefit from using a red torch.

If you are away from city lights altogether then a red torch is essential equipment.

If you are taking a group of students you should facilitate a discussion of dark adaptation and get them to make their own red torches.

Light Pollution

During the day the sun lights up the sky. The sky glows blue and overwhelms the other stars, which are all fainter than the blue of the sky.

At night streetlights light up the sky. The sky glows white and this overwhelms the fainter stars and the Milky Way. Only the brighter stars shine through. This effect is called light pollution.

Unlike the sun, streetlights do not illuminate the entire atmosphere. Their effect is limited to a dome-shaped region of air over each city. If you are in the city you see less stars and often cannot see the Milky Way at all. If you are out of the city, look back and you will see the effects of the streetlights as a dim white dome hovering over the city. This is called a light dome. Apart from the light dome, the rest of the sky will be almost totally dark and the number of stars visible will take your breath away.

Oddly enough, a dark sky does not always look any blacker than a light polluted sky. The difference with a dark sky is that you can see the Milky Way with exquisite clarity, and you can see thousands of faint stars.

How far do you have to get away before you can see a truly dark sky? That depends on atmospheric conditions, but you should notice a significant improvement in the sky by the time you are about five to ten miles (eight to fifteen kilometres) away from the edge of the city. By the time you are 20 miles (30 kilometres) away you should be able to look back and see the light dome. This is a sign that you are reasonably clear of the light dome – the Milky Way will look fantastic at this distance.

Although it is best to start stargazing in the city, consider making a trip to a dark skies location if you possibly can.

You could also lobby your local city authorities to take steps to reduce light pollution in your city. See the Dark Skies Association web site for more information about this.

Moon

Moonlight lights up the sky at night, and if it is bright enough it has a similar effect to light pollution – it stops you seeing the Milky Way and the faint stars. If you are planning a dark-sky session and hoping to see the Milky Way at its best, you need to go between last quarter and first quarter – a week either side of new moon. Check phases of the moon here.

Horizon

When selecting a star gazing site the ideal is to have a low horizon (no mountains or tall buildings) in all directions. On the top of a hill is often good for this. Unfortunately it is not always possible to find such an ideal site. Look at your Star Wheel to see which part of the sky contains the most interesting stars on the night you plan to go stargazing. Try to have a low horizon in at least that direction.

When assessing different sites you can visit them in daylight to look at the horizon.

Sunset Time and Season

If you live at latitude 45 or above you will find that it does not get properly dark until maybe 11pm in the summer. This may make it impractical to organise a group stargazing trip, especially for children.

In the winter you will find it is dark early enough, but is bitterly cold for stargazing.

In these latitudes the best time for stargazing is likely to be in spring before daylight saving has started and autumn (fall) after daylight saving has finished.

Of course, you can stargaze all year round. Check sunset times here. The brightest stars appear about 30 minutes after sunset, and it is usually sufficiently dark about one hour after sunset.

Weather

In some parts of the world you cannot guarantee a clear sky when you plan a stargazing session. How you manage this factor depends on your local climate. How many clear nights do you usually have?

In Dunedin, NZ, perfectly clear skies are comparatively rare. We get a lot of coastal cloud. If I want to plan a guaranteed clear dark-sky session I plan at least 16 postponement nights. I select only nights with the moon below the horizon during the stargazing session. I postpone the session at 9am in the morning unless the sky is perfectly clear at the stargazing place, and the forecast is for clear skies until at least noon the following day. Details of postponements are posted on the Internet so that all participants can check them during the day. If the sky and forecast are both still okay at sunset, then we travel for one hour by bus to get away from the Dunedin light dome. (Dunedin has a population of 100,000. Travel times may be longer from larger cities.) The result is a guaranteed jaw-dropping spectacle. Given the Dunedin microclimate, I can only be sure of delivering this wonderful experience by making these elaborate arrangements.

I hope you are blessed with a climate that delivers more clear nights so that your stargazing arrangements can be simpler. Depending on your climate, it may be just a matter of choosing a time of year when clear skies predominate.

Clothing

When you are stargazing, you do not move around very much. Unless conditions are genuinely tropical, you must make sure that all stargazers are well wrapped up in warm clothes. The need for gloves, hats, jackets, several pairs of trousers, and very warm socks cannot be over-emphasised.

Planets and Satellites

In Activity 6 – Planet Hit List you will learn how to predict which planets will be visible. It is valuable to know where the bright planets will be, and which of them will be visible during your stargazing session.

Just after dark is the best time for spotting satellites. It is a good idea to visit Heavens Above and find out which (if any) of the high-interest satellites like the International Space Station will be passing overhead during your stargazing session.

Binoculars and Telescopes

If you are organising a stargazing session for a group it is desirable to have as many pairs of binoculars available as possible. Some of the best targets for binoculars are described in Activity 8 – Jewels of the Sky.

If you have a telescope available, all the better, provided you have an experienced operator. With one or two telescopes a stargazing session can easily consist of standing in a queue most of the night. The solution to this is to turn queuing into an educational experience. Get those in the queue to locate stars and constellations with their Star Wheels. You can also pass a pair of binoculars up and down the queue.

What You Need

Working from the City Wheel, make a list of all the stars and constellations that will be visible during your stargazing session. You might also include some targets from the Milky Way Wheel, or, if you have binoculars available, some targets from Activity 8 – Jewels of the Sky.

Make this list into a kind of scavenger hunt as a way of keeping students focussed. You can also get students to work in pairs and sign one another off when they have demonstrated that they have found the target.

A sketching activity is a useful challenge that helps develop the skill of making drawings from observations. Give students extra copies of the City Wheel and get them to sketch the Milky Way on it, or some of the constellations that are not already shown on it.

Instructions

Every stargazing session is different so we cannot give specific instructions. If you have considered all of the above factors, then you are on your way to having a successful stargazing session.

Wishing you “Clear skies!”