{"id":37264,"date":"2022-10-20T00:00:00","date_gmt":"2022-10-20T00:00:00","guid":{"rendered":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/?post_type=purple_issue&p=37264"},"modified":"2023-03-30T09:04:31","modified_gmt":"2023-03-30T09:04:31","slug":"diy-astronomy-how-to-build-a-sextant","status":"publish","type":"post","link":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/2022\/10\/20\/diy-astronomy-how-to-build-a-sextant\/","title":{"rendered":"DIY Astronomy: How to build a sextant"},"content":{"rendered":"\n

Practical astronomy projects for every level of expertise <\/p>\n\n

DIY Astronomy <\/span><\/h2>\n\n
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How to build a sextant<\/h4>\n\n

Measure altitudes the ancient astronomers\u2019 way with this traditional navigation instrument<\/p>\n\n

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The completed sextant ready for some measuring. Use our simple downloadable guide to help with setting up<\/figcaption><\/figure><\/div>\n\n

This month\u2019s project is a sextant that you can use to measure the altitude of objects in the sky. At noon (taking care to use the built-in solar filter), you can measure the height of the Sun and use this to find your location on Earth. Our design uses two mirrors: the index mirror, mounted on a moveable pointer arm, and the fixed horizon mirror, which is half-clear, producing two images. In use, the left-hand image (usually of the horizon) is viewed through the clear side, while the right-hand image arrives via reflection in both mirrors (passing through a solar filter in between, when viewing the Sun). By adjusting the angle of the index mirror, you can align the image of the object you are measuring with the horizon on the left. The pointer arm then indicates the altitude of the object on a printed scale, measured in degrees above the horizon. <\/p>\n\n

The sextant is made from thin MDF. Our downloadable templates show the parts to cut out, along with a printable scale. We cut inexpensive craft mirrors to size with a glass cutter. This is a bit tricky, but a glass supplier may be able to cut these for you. Mask off one half of the back of the horizon mirror, then gently scrape away the painted layer and the silvering, taking care not to scratch the glass. Both mirrors are stuck to thin MDF backings that are held to the mirror mounts by M4 screws and nuts, with spongy foam spacers sandwiched between that allow them to be finely adjusted when setting up the instrument. Our downloadable guide explains how to set up. Once you get your sextant working, there are numerous online tutorials for using it.<\/p>\n\n

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MORE ONLINE
<\/strong>Download diagrams, a setup guide and photos to help with your build from www.skyatnightmagazine.com\/bonus-content\/GD67VIT<\/a><\/p>\n<\/div><\/section>\n\n

Where on Earth? <\/strong><\/h5>\n\n

If you can\u2019t see the horizon, you can still use your sextant. We achieved this by adding a long arm extending forwards, with an upright rod and a horizon indicator wire (a paperclip!) attached at the exact height of the eyepiece centreline. We stuck a piece of black tape to the clear side of the horizon mirror, also at this height. If you visually line up the wire and top of the tape, you are very close to horizontal. The indicator wire then serves as the horizon for measurements. Our guide also explains alternative methods. <\/p>\n\n

Using the sextant, we were able to measure when noon occurred for our location (we estimated approximately 12:08 UT) indicating a longitude of 2.0\u00b0 west. The height of the Sun measured 52\u00b0. We subtracted this from 90\u00b0 to find the latitude (adding on 12.3\u00b0 correction for the date, which we looked up at thenauticalalmanac.com<\/a>). Our calculated position was only about 100km from our actual location \u2013 not too bad considering the resolution of the scale and the overall size of our planet! <\/p>\n\n

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What you\u2019ll need <\/strong><\/h5>\n\n\n\n

Tools: ruler, compasses, pencil, coping saw, junior hacksaw, drill and drill bits, glass cutter, craft knife blade. <\/p>\n\n\n\n

Two pieces of approximately A4-sized thin MDF or plywood, a pair of approved solar eclipse glasses, two craft mirrors 50 x 35mm, eight M4 x 20 screws, nuts and washers, a piece of spongy foam. <\/p>\n\n\n\n

Piece of thin tube approximately 20mm diameter to make the eyepiece. <\/p>\n\n\n\n

Spray paint, glue.<\/p>\n<\/div><\/section>\n\n

Step by step <\/strong><\/h4>\n\n
Step 1 <\/strong><\/h5>\n\n
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Carefully mark out the MDF: print out the templates for the main parts (be sure to set your printer scale to 100% for all printouts), cut them out and glue them on to the MDF. Cut out all the parts with a coping saw, or similar, and sand the edges smooth. <\/p>\n\n

Step 2 <\/strong><\/h5>\n\n
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Carefully drill the holes in the main panel and the mirror mount parts. Use a 4mm drill for pivots but a 4.5mm drill bit for the mounts, so that the angle of each mirror can be adjusted a little, relative to the mount. <\/p>\n\n

Step 3 <\/strong><\/h5>\n\n
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Glue together the two parts of each mirror mount and leave to dry. You can also glue on the eyepiece parts and the support for the solar filter. Painting the wooden parts makes them look nice but also provides a bit of long-term protection. <\/p>\n\n

Step 4 <\/strong><\/h5>\n\n
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Print off the scale and stick directly to the main panel. Laminate it first if possible. Alternatively, use a protractor to mark out your own accurate scale (or even stick a protractor on), but note that at the scale we\u2019ve used for this build the 10\u00b0 markings are 5\u00b0 apart. <\/p>\n\n

Step 5 <\/strong><\/h5>\n\n
\"\"<\/figure><\/div>\n\n

Cut the mirrors to size, being careful of sharp edges. Use tape to mask half the back of one. Cut through the paint backing along the line, then gently scrape away the paint and silvering with a blade. Metal polish can be used to remove any stubborn bits. <\/p>\n\n

Step 6 <\/strong><\/h5>\n\n
\"\"<\/figure><\/div>\n\n

Glue the mirrors to their backings and cut one half of the eclipse glasses to make the filter. Assemble all the parts, using foam spacers between mirror backings and mounts so they can be adjusted. Use our downloadable guide to set up the instrument. <\/p>\n\n

<\/p>\n\n

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<\/figcaption><\/figure><\/div>\n\n

Mark Parrish is an amateur astronomer and bespoke designer based in West Sussex.<\/p>\n\n

<\/p>\n\n

ALL PICTURES: MARK PARRISH<\/p>\n","protected":false},"excerpt":{"rendered":"

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