{"id":92,"date":"2018-01-16T17:36:33","date_gmt":"2018-01-16T17:36:33","guid":{"rendered":"http:\/\/www.popastro.com\/main_spa1\/youngstargazers\/?page_id=92"},"modified":"2021-04-01T22:49:26","modified_gmt":"2021-04-01T21:49:26","slug":"make-your-own-sundial","status":"publish","type":"page","link":"https:\/\/www.popastro.com\/youngstargazers\/make-your-own-sundial\/","title":{"rendered":"Make your own sundial"},"content":{"rendered":"

You may think that sundials are little more than garden decorations, usually telling the wrong time. That’s true of many, because they are badly designed, but you can easily make a sundial that is far more accurate using materials you can probably find in the home. It has the potential to be accurate to the minute. You can use our sundial anywhere in Europe or the US, and you can even set it to tell summer time.
\n\"sundial\"<\/p>\n

You will need:<\/h2>\n

An old CD or, ideally, a transparent CD-sized disc as used in spindles of blank CDs or DVDs
\nAn empty CD case
\nA cocktail stick
\nBlu-tack or similar
\nSticky tape (ideally 25 mm wide)
\nTwo small circular pieces of cardboard
\nA protractor
\nThe hour circle<\/a>\u00a0(click this link to download a PDF file)<\/p>\n

\"sundial\"<\/p>\n

How it works<\/h2>\n

Every sundial has a shadow stick or gnomon that should be set parallel to the Earth’s axis. The Sun’s movement through the sky will then be at right angles to the gnomon, so a scale at right angles to the gnomon can be marked with equal hour lines. Most garden sundials have scales that are at a different angle, so the hour lines are not equal. This makes it impossible to correct them for summer time. We have supplied you with a scale of equal hour lines, designed to fit exactly around the edge of a 12 cm CD. You must print this out to exactly 188.5 mm across so that it will fit. Print out the PDF file and it should be the right size: adjust ‘Page Setup’ of Adobe Reader to Landscape to avoid problems with page margins.<\/p>\n

Making your sundial<\/h2>\n

Step 1<\/b>\u00a0Print off and cut out the hour scale.<\/p>\n

Step 2<\/b>\u00a0Apply sticky tape to the back of the scale so that the tape overlaps the bottom of the scale by about 3 mm.<\/p>\n

Step 3<\/b>\u00a0Stick the scale to the edge of the CD with the scale facing inwards by rolling the CD along the sticky overlapping edge.<\/p>\n

\"sundial\"<\/p>\n

Step 4\u00a0<\/b>Stick two circles of thin card to each side of the central hole in the CD. Tip \u2013 party poppers have discs of exactly the right size!<\/p>\n

Step 5\u00a0<\/b>Make a hole in the exact centre of the CD with a sharp point, then push the cocktail stick through at right angles to the CD. Tip \u2013 if you don’t have a cocktail stick, straighten out a paper clip instead.<\/p>\n

\"\"<\/p>\n

Step 6<\/b>\u00a0Use a blob of Blu-tack to fix the CD at an angle within the CD case as shown, with the noon or 1 pm hour at the bottom depending on whether GMT or BST* (often called Daylight Saving Time) is in force. Use a protractor to make sure that the gnomon is at the same angle to the horizontal as your latitude (between 50\u00ba and 59\u00ba depending on where you live in the UK*). That’s it: your sundial is ready for use.<\/p>\n

\"sundial\"<\/p>\n

\"sundial\"<\/p>\n

* The sundial was designed to be used in the UK, where Greenwich Mean Time or British Summer Time are used. But you can use the sundial anywhere in the northern hemisphere as well by tilting the gnomon at an angle equal to your latitude. There may also be a correction to the time depending on where you are in your time zone. See the question about longitudes below.<\/p>\n

Using your sundial<\/h2>\n

Point the gnomon due north in a sunny spot. Alternatively, as it can be hard to find north indoors, if you have made the sundial correctly you can align it so that the shadow of the gnomon falls on the scale indicating the correct time, and it should then be fairly accurately set.<\/p>\n

\"sundial\"<\/p>\n

The dial will give you solar time, which usually differs from clock time by an amount that varies throughout the year. The difference is known as the Equation of Time. Here is a table giving the approximate dates when the sundial is fast (\u2013) or slow (+) compared with your watch by a whole number of minutes:<\/p>\n\n\n\n
Jan 1\u00a0\u00a0\u00a0 3
\nJan 3\u00a0\u00a0\u00a0 4
\nJan 5\u00a0\u00a0\u00a0 5
\nJan 8\u00a0\u00a0\u00a0 6
\nJan 10\u00a0\u00a0 7
\nJan 12\u00a0\u00a0 8
\nJan 15\u00a0\u00a0 9
\nJan 18 10
\nJan 2 \u00a0 11
\nJan 2\u00a0\u00a0 12<\/small><\/td>\n		\n
Jan 29\u00a0\u00a0 13
\nFeb 5\u00a0\u00a0\u00a0 14
\nFeb 26\u00a0 13
\nMar 4\u00a0\u00a0\u00a0 12
\nMar 8\u00a0\u00a0\u00a0 11
\nMar 2\u00a0\u00a0\u00a0 10
\nMar 16\u00a0\u00a0\u00a0 9
\nMar 19\u00a0\u00a0\u00a0 8
\nMar 22\u00a0\u00a0\u00a0 7
\nMar 26\u00a0\u00a0\u00a0 6<\/small><\/div>\n<\/td>\n
\n
Mar 29\u00a0\u00a0 5
\nApr 1\u00a0\u00a0\u00a0\u00a0\u00a0 4
\nApr 5\u00a0\u00a0\u00a0\u00a0\u00a0 3
\nApr 8\u00a0\u00a0\u00a0\u00a0\u00a0 2
\nApr 12\u00a0\u00a0\u00a0 1
\nApr 16\u00a0\u00a0\u00a0 0
\nApr 21\u00a0 \u20131
\nApr 26\u00a0 \u20132
\nMay 3\u00a0 \u2013 3
\nJun 3\u00a0\u00a0 \u2013 2<\/small><\/div>\n<\/td>\n
\n
Jun 9\u00a0\u00a0\u00a0\u00a0\u00a0 \u20131
\nJun 14\u00a0\u00a0\u00a0 \u20130
\nJun 19\u00a0\u00a0\u00a0\u00a0\u00a0 1
\nJun 24\u00a0\u00a0\u00a0\u00a0\u00a0 2
\nJun 29\u00a0\u00a0\u00a0\u00a0\u00a0 3
\nJul 4\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 4
\nJul 10\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 5
\nJul 18\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 6
\nAug 12\u00a0\u00a0\u00a0\u00a0 5
\nAug 17\u00a0\u00a0\u00a0\u00a0 4<\/small><\/div>\n<\/td>\n
\n
Aug 22\u00a0\u00a0\u00a0\u00a0 3
\nAug 26\u00a0\u00a0\u00a0\u00a0 2
\nAug 29\u00a0\u00a0\u00a0\u00a0 1
\nSep 2\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 0
\nSep 5\u00a0\u00a0\u00a0\u00a0 \u20131
\nSep 8\u00a0\u00a0\u00a0\u00a0 \u20132
\nSep 11\u00a0\u00a0 \u20133
\nSep 14\u00a0\u00a0 \u20134
\nSep 17\u00a0\u00a0 \u20135
\nSep 20\u00a0\u00a0 \u20136<\/small><\/div>\n<\/td>\n
\n
Sep 22\u00a0 \u20137
\nSep 25\u00a0 \u20138
\nSep 28\u00a0 \u20139
\nOct 1\u00a0\u00a0\u00a0 \u201310
\nOct 4\u00a0\u00a0\u00a0 \u201311
\nOct 8\u00a0\u00a0\u00a0 \u201312
\nOct 11\u00a0 \u201313
\nOct 15\u00a0 \u201314
\nOct 20\u00a0 \u201315
\nOct 27\u00a0 \u201316<\/small><\/div>\n<\/td>\n
Nov 17\u00a0 \u201315
\nNov 22\u00a0 \u201314
\nNov 25\u00a0 \u201313
\nNov 28\u00a0 \u201312
\nDec 1\u00a0\u00a0\u00a0 \u201311
\nDec 4\u00a0\u00a0\u00a0 \u201310
\nDec 6\u00a0\u00a0\u00a0\u00a0\u00a0 \u20139
\nDec 8\u00a0\u00a0\u00a0\u00a0\u00a0 \u20138
\nDec 11\u00a0\u00a0\u00a0 \u20137
\nDec 13\u00a0\u00a0\u00a0 \u20136<\/small><\/td>\n		Dec 15\u00a0\u00a0\u00a0 \u20135
\nDec 17\u00a0\u00a0\u00a0 \u20134
\nDec 19\u00a0\u00a0\u00a0 \u20133
\nDec 21\u00a0\u00a0\u00a0 \u20132
\nDec 23\u00a0\u00a0\u00a0 \u20131
\nDec 25\u00a0\u00a0\u00a0\u00a0\u00a0 0
\nDec 28\u00a0\u00a0\u00a0\u00a0\u00a0 1
\nDec 30\u00a0\u00a0\u00a0\u00a0\u00a0 2<\/small><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

These figures apply wherever you are in the world or whatever your time zone. However, your location within your time zone \u2013 in the UK, how far east or west of the Greenwich meridian \u2013 also affects the sundial reading. \u00a0See the questions below.<\/p>\n

Questions for you to think about<\/h3>\n

Q\u00a0Why is it best that the CD is transparent?<\/strong><\/p>\n

A\u00a0Think about what happens in winter. The Sun gets low in the sky, and in fact between about September and March 21 it is below what’s called the celestial equator. The CD is parallel to this, so in winter sunlight falls on the lower side of the CD. If it is not transparent, the hour circle will be in the shadow of the CD so you won’t be able to see the shadow of the gnomon. In real armillary sundials there is no structure to get in the way, so they work at all times of year (see below).<\/p>\n

Q\u00a0Over what range of latitudes will your sundial work?<\/strong><\/p>\n

A\u00a0Its range is limited by the amount over which you can tilt the gnomon in the CD case. If you can find a different way to support the CD at an angle, it will work literally anywhere in the world. At the Equator the CD would have to be vertical and the gnomon horizontal. In the southern hemisphere the hour circle would be upside down, but it will still work.
\n
Q\u00a0Over what range of longitudes will your sundial work?<\/strong><\/p>\n

A\u00a0All of them. Local time depends on the location of the Sun in the sky, though where you are in your time zone also plays a part. For example, the whole of the British Isles are within the same time zone. However, when the Sun is overhead at Greenwich (on which the time zone is based), it is still 40 minutes away from being overhead in the west\u00a0of Ireland. There, noon\u00a0occurs not at 12.00 but 12.40 in winter, or 1.40 pm in summer.<\/p>\n

You can set your sundial to read civil time wherever you are by fixing it to the CD case so that the bottom point allows for your time zone difference. So in Plymouth, Swansea or Glasgow, for example, which are all about 4\u00ba west of Greenwich, the bottom point would read not 12.00 but 12:16 in winter and 1:16 in summer. The 16 minute difference is additional to\u00a0the Equation of Time. So to make your sundial super-accurate, you would reset it every few days to take into account both effects.<\/p>\n

So for example, to allow for the Equation of Time on December 4 (-10 minutes on the table) you would set the bottom point of the hour circle to be not 12.00 but 11.50. But for every degree you are west of Greenwich, you need to add four minutes of time to the setting, so in Glasgow you would actually set the bottom point to 11:54.<\/p>\n

Outside the UK, there is sometimes a big difference between solar time as given by the sundial and local time.
\n
Q\u00a0Why does the hour scale stop at 6 am and 6 pm?<\/strong><\/p>\n

A\u00a0Because otherwise the shadow of the hour circle itself would obscure times in the early morning and early evening around March and September. You could renumber and skew the circle to tell whatever range of times you choose within a 12-hour period.
\n
Q\u00a0Why does solar time often differ from civil time?<\/strong><\/p>\n

A\u00a0The reason for the Equation of Time is that the Earth’s orbit around the Sun is not a circle but an ellipse, and it moves faster when it is close to the Sun than when it is far away. But unequal hours are not good for civil timekeeping, so we even out the differences over the year.<\/p>\n

Q\u00a0Why have you got IIII instead of IV for 4 pm ?<\/strong><\/p>\n

A\u00a0That’s the way clock faces are traditionally drawn. You can research this one on the Internet for yourself by entering, for example, ‘roman IIII 4 pm’ into a search engine. Use four capital letter I.<\/p>\n","protected":false},"excerpt":{"rendered":"

You may think that sundials are little more than garden decorations, usually telling the wrong time. That’s true of many, because they are badly designed, but you can easily make a sundial that is far more accurate using materials youRead more<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"parent":0,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"_monsterinsights_skip_tracking":false,"footnotes":""},"categories":[],"tags":[],"class_list":["post-92","page","type-page","status-publish","hentry"],"aioseo_notices":[],"aioseo_head":"\n\t\t\n\t\n\t\n\t\n\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t