{"id":42665,"date":"2023-03-23T09:52:45","date_gmt":"2023-03-23T09:52:45","guid":{"rendered":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/?post_type=purple_issue&p=42665"},"modified":"2023-03-23T10:15:15","modified_gmt":"2023-03-23T10:15:15","slug":"the-sky-guide-challenge-19","status":"publish","type":"post","link":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/2023\/03\/23\/the-sky-guide-challenge-19\/","title":{"rendered":"The Sky Guide challenge"},"content":{"rendered":"\n

Visually demonstrate the two types of solar time with this year-long project<\/p>\n\n

A sundial is an easy way to measure time. You simply put a stick in the ground and mark where the end of the stick\u2019s shadow falls on the hour. Not too much of a challenge you might think, but there are hidden details you need to consider. <\/p>\n\n

The trouble is, the sundial\u2019s measured daily time starts to drift with respect to clock time. Time will not always be shown accurately, a small difference at first, then larger, then smaller, then zero and so on. The problem isn\u2019t with your sundial, it\u2019s with the Sun, or rather with Earth. Earth\u2019s orbit about the Sun isn\u2019t a circle, it\u2019s an ellipse. As we travel around the Sun, the elliptical orbit causes the Earth\u2013Sun distance and our orbital speed to alter. For us sitting on the surface of our planet, the apparent speed of the Sun across the sky appears to speed up and slow down in a periodic manner. We can\u2019t gauge time properly using this variable Sun, known as the real or apparent Sun, so we create an imaginary one which moves across the sky at a consistent rate: the mean Sun. <\/p>\n\n

The mean and apparent Sun align on four dates throughout the year: this month on 15 April, then 13 June, 1 September and 25 December (correct for J2000.0). At such times, the end of the shadow of our sundial stick (gnomon) marks the correct time. On other days the mean Sun is either ahead of or behind the apparent Sun. The difference is known by the rather enigmatic name of \u2018the equation of time\u2019 and can range from 14 minutes and 15 seconds ahead of clock time to 16 minutes and 25 seconds behind.<\/p>\n\n

The variation can be demonstrated to great effect by photographing the position of the Sun over the course of a year at the same UT time. If you do this, the equation of time causes it to oscillate back and forth. However, Earth\u2019s axial tilt also plays a part, causing the Sun to appear to move up and down in the sky over the course of a year. As a consequence, the equation of time plus declination drift produces a bowling pin shape in the sky, known as an analemic curve. <\/p>\n\n

Marking the position of a gnomon\u2019s shadow tip at the same time of day throughout the year will also produce this curve, and that\u2019s your challenge for the rest of 2023 and into 2024! This requires good placement of the gnomon so it\u2019ll be in Sun throughout the year. It also requires good weather. Don\u2019t worry about missing days though \u2013 the shape can still be revealed, even with many gaps. <\/p>\n\n

\"\"
Starting on 15 April, track the discrepancy between mean and apparent solar times by marking where a gnomon\u2019s shadow falls at the same time each day <\/figcaption><\/figure><\/div>\n\n

PHOTO AND GRAPH: PETE LAWRENCE<\/p>\n","protected":false},"excerpt":{"rendered":"

Visually demonstrate the two types of solar time with this year-long project <\/p>\n","protected":false},"author":24,"featured_media":43181,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"ub_ctt_via":"","purple_page_number":"55","purple_custom_meta_purple_page_number":"55","purple_seq_number":"1","purple_custom_meta_purple_seq_number":"1","purple_source_article":"article_55-1.xml","purple_custom_meta_purple_source_article":"article_55-1.xml","purple_source_issue":"April-2023","purple_custom_meta_purple_source_issue":"April-2023","purple_external_id":"April-2023-55-1","purple_custom_meta_purple_external_id":"April-2023-55-1","purple_issue_code":"|0000086563||","purple_custom_meta_purple_issue_code":"|0000086563||","purple_android_product":"com.im.skyatnight.215","purple_custom_meta_purple_android_product":"com.im.skyatnight.215","purple_ios_product":"com.im.skyatnight.215","purple_custom_meta_purple_ios_product":"com.im.skyatnight.215","purple_web_product":"","purple_custom_meta_purple_web_product":"","purple_publication_id":"075fab74-0a21-4201-866a-899d6c41c40c","purple_migrated":"","kt_blocks_editor_width":""},"categories":[22],"tags":[14],"featured_image_src":"https:\/\/c01.purpledshub.com\/uploads\/sites\/77\/2023\/03\/3-2.jpg","author_info":{"display_name":"importmanagerhub@sprylab.com","author_link":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/author\/importmanagerhubsprylab-com\/"},"acf":{"readingTimeMinutes":"3","apple_news_title":""},"uagb_featured_image_src":{"full":["https:\/\/c01.purpledshub.com\/uploads\/sites\/77\/2023\/03\/3-2.jpg",2126,1657,false],"thumbnail":["https:\/\/c01.purpledshub.com\/uploads\/sites\/77\/2023\/03\/3-2-150x150.jpg",150,150,true],"medium":["https:\/\/c01.purpledshub.com\/uploads\/sites\/77\/2023\/03\/3-2-300x234.jpg",300,234,true],"medium_large":["https:\/\/c01.purpledshub.com\/uploads\/sites\/77\/2023\/03\/3-2-768x599.jpg",768,599,true],"large":["https:\/\/c01.purpledshub.com\/uploads\/sites\/77\/2023\/03\/3-2-1024x798.jpg",800,623,true],"1536x1536":["https:\/\/c01.purpledshub.com\/uploads\/sites\/77\/2023\/03\/3-2-1536x1197.jpg",1536,1197,true],"2048x2048":["https:\/\/c01.purpledshub.com\/uploads\/sites\/77\/2023\/03\/3-2-2048x1596.jpg",2048,1596,true]},"uagb_author_info":{"display_name":"importmanagerhub@sprylab.com","author_link":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/author\/importmanagerhubsprylab-com\/"},"uagb_comment_info":0,"uagb_excerpt":"Visually demonstrate the two types of solar time with this year-long project","_links":{"self":[{"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/posts\/42665"}],"collection":[{"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/users\/24"}],"replies":[{"embeddable":true,"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/comments?post=42665"}],"version-history":[{"count":5,"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/posts\/42665\/revisions"}],"predecessor-version":[{"id":43439,"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/posts\/42665\/revisions\/43439"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/media\/43181"}],"wp:attachment":[{"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/media?parent=42665"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/categories?post=42665"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/tags?post=42665"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}