Introduction

In this experiment you will make your own observations of the Southern Sky, and use them to determine the latitude at which you live. This investigation is designed to give you an understanding of how the stars rotate across the night sky, and also of how the Earth rotates and is oriented in space. Equipment Assemble this equipment before you start the exercise: • A long, straight piece of cardboard, a metre ruler, or a straight straw • A protractor • A length of string with a weigh on one end • Paper and a pen or pencil • A camera capable of taking long exposures or sheets of paper to draw the night sky Risk Assessment Read over the experimental method and then complete a risk assessment in the table below. You must minimize all risks before you start. Add as many rows to the table as you need. If any of the risks rank above medium then you need to adapt the experiment to make it safer before proceeding. Risk rating with existing controls * Tasks (Step 5) Hazards (Step 3) Associated risks (Step 4) Existing risk controls C L R By proceeding with the experiment you are agreeing to follow these risk control methods and conduct the experiment safely. If you have any doubts about your ability to complete the experiment safely then you should not proceed. In your report you need to state that you agree to follow these risk controls. Without this statement your report will not be marked. Procedure 1. Build a clinometer (http://en.wikipedia.org/wiki/Inclinometer). This device will allow you to determine how far above the horizon something is. A diagram of a simple clinometer can be found at http://mymathblog.wikispaces.com/file/view/374.gif/302080980/374.gif. 2. Go outside on a clear night to a location from which you have a clear view of the southern sky. Note: The sky should be fully dark at the time you begin your observations – at least 60 minutes after sunset, but preferably 90 minutes. 3. Mark your observing location, so that you can return to it at hourly intervals. Then allow yourself five minutes to become somewhat adapted to the darkness (it takes well over half an hour to become truly dark adapted – but you can get somewhat adapted in just five minutes). 4. If you have a camera capable of taking long exposures, set it up with a wide-angled lens, pointed towards the south, somewhere between 30 and 45 degrees above the horizon. 5. Take a long exposure with your camera (preferably 30s in duration). If you are setting your camera manually, ensure that the focal length (f-number) is set to be as small/fast as possible (i.e. f3.5 or f2 rather than f16 or f20!). Remember to focus so that the stars are visible, if you are using manual focus! 6. If you are not using a camera, sketch the brightest stars in the southern sky on your paper, as accurately as possible. Include the horizon and any landmarks that you can see, that will help you to re-orientate yourself when you return. 7. Repeat the above procedure after one hour, two hours and three hours have elapsed. You should now have four photographs or sketches, and the position of the stars should have moved from one to the next. 8. You are now ready to determine the location of the south celestial pole. Each star you have drawn is moving on a circular path around the south pole in the sky. They complete one full lap in a time period of 23 hours 56 minutes and 4 seconds (we’ll call it 24 hours from now on, to keep things simple). Use your images to work out which point on the sky the stars are moving around. 9. Once you have determined the point in the sky about which you think the stars are rotating, use your home-made clinometer to measure the angular height (the altitude) of that point from the horizon. Results and Analysis: Step 1 Describe how you constructed your clinometer. Include a picture of your finished clinometer. How accurately do you think the clinometer will allow you to measure the altitude of a given target above the horizon? Explain your reasoning (hint: the clinometer may be less precise than simply taking half of the smallest gradiation on your protractor). Step 2 Four photographs taken, or sketches made, at hourly intervals of the southern sky. Make sure to include all of your pictures in your report. Step 3 Location of the south celestial pole determined Provide a final figure (either a sketch or a photograph) showing your proposed location for the south celestial pole. Step 4 Altitude of the south celestial pole measured with the clinometer. State your measured altitude, along with its uncertainty, in degrees. Questions: 1. Given that your observations span a period of three hours, through what angle will the stars you have observed have rotated around the celestial pole (assuming 1 day = 24 hours). 2. Explain, with the aid of a diagram, how the measurements you have made allow you to determine the latitude of your observing location. 3. How well did your measured value for your latitude agree with the literature value? 4. What steps could be taken to improve the precision with which you measured your latitude? 5. The Earth’s Arctic and Antarctic circles lie a distance of ~23.5 degrees away from the poles (roughly at latitudes 66.5 degrees North and South, respectively). Within the circles, places experience periods of 24 hour night-time (in the winter) and 24 hour daylight (in the summer). Explain this phenomenon, with the aid of a diagram. What to submit: Submit a report through TurnItIn on Moodle. The report should include: • A statement of the aim of the investigation • A statement saying that you will follow the risk controls recommended in the risk assessment • Everything in the results and analysis section of the investigation • Details of your analysis of the results and predictions • Answers to the questions • A conclusion stating what you have found out You report should not include: • The procedure (as this has been given to you, only comment if you make a change to this procedure) You may write your report neatly by hand and scan it in (there are apps for iphones and ipads that allow you to scan documents in) or type-set it. It can take a long time to typeset mathematical expressions so the first option may be a lot faster. Rubric (this rubric has been programmed into TurnItIn): Criteria Maximum possible marks Identified at least two tasks with hazards, associated risks and suitable controls identified. Rated these risks and controls using codes in “Rating Risks” document 2 Photograph of final clinometer presented 1 Reasonable estimate of clinometer’s precision (with reasoning) 1 First photograph or sketch taken/made, with at least 10 stars marked/shown 1 Four photographs or sketches shown, with at least 10 stars 1 Location of South Celestial Pole determined (with picture) 1 Altitude of South Celestial Pole measured, with uncertainty and units 1 Answered question 1 1 Answered question 2 2 Answered question 3 1 Answered question 4 1 Answered question 5 2 TOTAL: 15 (This contributes 5% towards your total mark for the subject