Greenhenge

posted: 441 days ago, on Tuesday, 2016 Mar 08 at 20:27
tags: astronomy, ethnoastronomy, Sun, Earth, events, Cape Town.

Introduction

Stonehenge in Wiltshire, England, is one of several ancient structures aligned with the rising Sun. At the time of the summer solstice, the Sun rises over a particular stone marker as seen from the centre of the structure.

Several factors disincline the writer’s desire to participate in such a celebration. Stonehenge is far away, June 21 is the writer’s birthday, and sunrise is an ungodly hour. Fortunately, there are other alternatives.

In Manhattan, a borough of New York City in the United States, one may encounter Manhattanhenge. Due to the regular grid structure of the streets, which are aligned east-west, it is possible to look down the length of 42nd Street at certain times of the year and see the Sun setting between the profiles of the tall buildings. Similarly, one finds Baltimorehenge, Chicagohenge, Torontohenge, Montrealhenge, and MIThenge. The latter is somewhat different: here, the setting Sun is viewed down the length of the Infinite Corridor (a 251-m hallway) on the campus of the Massachusetts Institute of Technology.

While presenting more favourable viewing times, these venues are also not suitable: not only are they further away than Stonehenge, one is highly likely to encounter noiseome Americans. Is there a closer venue, the writer wondered? Specifically, can there be an as-yet undiscovered henge in Cape Town?

Figure 1 shows the Google Earth view of central Cape Town. The diagrams have north at the top. The city collars Signal Hill, and portions eastward are robbed of a setting Sun. Westward, development follows the contours of the topography, trending northeast-southwest. However, north of Signal Hill, and running roughly east-west, is the 1-km Main Road (1B). Could this be Greenhenge?

Greenhenge?

Figure 2 shows that it should be possible to see the Sun setting along Main Road, appearing briefly between two tall buildings, from a viewing point at the easternmost section of Main Road (before it bends southwards). The “Greenhenge Viewing Point” is at [33°54’26.85”S, 18°24’28.66”E].

The two buildings are known as the Twin Towers, a 16-floor (~59-m tall) residential complex on the corner of Beach and Fort Roads[1]. Figure 3 shows them from a closer point, as well as a 6-floor foreground building (corner Main and Three Anchor Bay Roads).

From the viewing point the distance to the Twin Towers is 1.13-km and to the foreground building, 0.98-km.

Viewing chances

From the Greenhenge Viewing Point, the Twin Towers stand 3° tall; the distance between them is estimated as 2°. The foreground building obscures the actual horizon by an estimated 1.5°, giving a “viewing portal” of perhaps 2° x 1.5°.

Using the Ruler Tool in Google Earth, the bearing from the viewing point to the centre of the viewing portal is 265.4° (average of four measures).

Assuming that Google Earth’s “bearing” is the same as astronomical azimuth, a table of date/time of sunset and azimuth of sunset was calculated (Appendix; Table 1). For each value, the difference between azimuth at sunset and “bearing” was determined; dates when these absolute differences are smaller than 1° are highlighted in the table, and are as follows:

Table 2. Dates for Greenhenge

DateSASTazimuthd-azimuth
 
 
Mar 09 Wed19:11:54264.5°0.9°
Mar 10 Thu19:10:35264.90.5
Mar 11 Fri19:09:15265.40.0
Mar 12 Sat19:07:55265.90.5
Mar 13 Sun19:06:35266.41.0
Sep 29 Thu18:47:58266.10.7
Sep 30 Fri18:48:41265.60.2
Oct 01 Sat18:49:25265.20.2
Oct 02 Sun18:50:09264.70.7
 

Invitation

Browsing Google Earth in Street View confirms that the viewing spot is rather akwardly located. There is a narrow central paved area where one could set up, but then robots (probably) block the view. Just 160-m further along Main Road is a pedestrian crossing and bus stop (opposite Waterfront Suites nogal) shown in Figure 4. Perhaps the short section of road, marked with an X in the figure, could be a strategic spot?

Who’s joining me for the first Greenhenge Celebration?

Acknowledgements

Thanks to X and Y for checking my calculations.

References

[1] http://www.emporis.com/complex/105185/twin-towers-cape-town-south-africa

Appendix

Table 1. Table 1. Moment of sunset, and azimuth, 2016, for Greenhenge

2016 dateSASTazimuth
 
 
Jan01 Fri20:00:43241.27
Jan02 Sat20:00:54241.38
Jan03 Sun20:01:03241.5
Jan04 Mon20:01:10241.63
Jan05 Tue20:01:15241.76
Jan06 Wed20:01:19241.91
Jan07 Thu20:01:20242.07
Jan08 Fri20:01:20242.23
Jan09 Sat20:01:17242.41
Jan10 Sun20:01:13242.59
Jan11 Mon20:01:07242.78
Jan12 Tue20:00:59242.98
Jan13 Wed20:00:49243.19
Jan14 Thu20:00:37243.41
Jan15 Fri20:00:24243.63
Jan16 Sat20:00:08243.87
Jan17 Sun19:59:51244.11
Jan18 Mon19:59:32244.36
Jan19 Tue19:59:11244.62
Jan20 Wed19:58:48244.89
Jan21 Thu19:58:23245.16
Jan22 Fri19:57:57245.44
Jan23 Sat19:57:28245.73
Jan24 Sun19:56:59246.03
Jan25 Mon19:56:27246.33
Jan26 Tue19:55:54246.64
Jan27 Wed19:55:19246.96
Jan28 Thu19:54:42247.28
Jan29 Fri19:54:04247.61
Jan30 Sat19:53:24247.95
Jan31 Sun19:52:42248.29
Feb01 Mon19:51:59248.64
Feb02 Tue19:51:15248.99
Feb03 Wed19:50:29249.35
Feb04 Thu19:49:41249.72
Feb05 Fri19:48:52250.09
Feb06 Sat19:48:02250.47
Feb07 Sun19:47:10250.85
Feb08 Mon19:46:17251.23
Feb09 Tue19:45:23251.63
Feb10 Wed19:44:27252.02
Feb11 Thu19:43:30252.42
Feb12 Fri19:42:32252.83
Feb13 Sat19:41:33253.24
Feb14 Sun19:40:32253.65
Feb15 Mon19:39:31254.07
Feb16 Tue19:38:28254.49
Feb17 Wed19:37:24254.91
Feb18 Thu19:36:19255.34
Feb19 Fri19:35:13255.78
Feb20 Sat19:34:07256.21
Feb21 Sun19:32:59256.65
Feb22 Mon19:31:50257.09
Feb23 Tue19:30:41257.53
Feb24 Wed19:29:30257.98
Feb25 Thu19:28:19258.43
Feb26 Fri19:27:07258.88
Feb27 Sat19:25:54259.34
Feb28 Sun19:24:41259.8
Feb29 Mon19:23:27260.25
Mar01 Tue19:22:12260.72
Mar02 Wed19:20:57261.18
Mar03 Thu19:19:41261.64
Mar04 Fri19:18:24262.11
Mar05 Sat19:17:07262.58
Mar06 Sun19:15:50263.05
Mar07 Mon19:14:31263.52
Mar08 Tue19:13:13263.99
Mar09 Wed19:11:54264.46
Mar10 Thu19:10:35264.93
Mar11 Fri19:09:15265.41
Mar12 Sat19:07:55265.88
Mar13 Sun19:06:35266.36
Mar14 Mon19:05:14266.83
Mar15 Tue19:03:54267.31
Mar16 Wed19:02:33267.78
Mar17 Thu19:01:12268.26
Mar18 Fri18:59:50268.74
Mar19 Sat18:58:29269.21
Mar20 Sun18:57:08269.69
Mar21 Mon18:55:46270.17
Mar22 Tue18:54:25270.64
Mar23 Wed18:53:03271.11
Mar24 Thu18:51:41271.59
Mar25 Fri18:50:20272.06
Mar26 Sat18:48:59272.53
Mar27 Sun18:47:37273
Mar28 Mon18:46:16273.47
Mar29 Tue18:44:55273.94
Mar30 Wed18:43:35274.41
Mar31 Thu18:42:14274.88
Apr01 Fri18:40:54275.34
Apr02 Sat18:39:34275.8
Apr03 Sun18:38:15276.26
Apr04 Mon18:36:55276.72
Apr05 Tue18:35:36277.18
Apr06 Wed18:34:18277.63
Apr07 Thu18:33:00278.09
Apr08 Fri18:31:42278.54
Apr09 Sat18:30:25278.98
Apr10 Sun18:29:09279.43
Apr11 Mon18:27:52279.87
Apr12 Tue18:26:37280.31
Apr13 Wed18:25:22280.75
Apr14 Thu18:24:08281.18
Apr15 Fri18:22:54281.62
Apr16 Sat18:21:41282.04
Apr17 Sun18:20:29282.47
Apr18 Mon18:19:18282.89
Apr19 Tue18:18:07283.31
Apr20 Wed18:16:57283.72
Apr21 Thu18:15:48284.13
Apr22 Fri18:14:40284.54
Apr23 Sat18:13:32284.94
Apr24 Sun18:12:26285.34
Apr25 Mon18:11:20285.73
Apr26 Tue18:10:15286.13
Apr27 Wed18:09:12286.51
Apr28 Thu18:08:09286.89
Apr29 Fri18:07:08287.27
Apr30 Sat18:06:07287.64
May01 Sun18:05:08288.01
May02 Mon18:04:09288.37
May03 Tue18:03:12288.73
May04 Wed18:02:16289.08
May05 Thu18:01:21289.43
May06 Fri18:00:27289.77
May07 Sat17:59:35290.1
May08 Sun17:58:43290.44
May09 Mon17:57:53290.76
May10 Tue17:57:05291.08
May11 Wed17:56:17291.39
May12 Thu17:55:31291.7
May13 Fri17:54:47292
May14 Sat17:54:03292.3
May15 Sun17:53:21292.58
May16 Mon17:52:41292.86
May17 Tue17:52:02293.14
May18 Wed17:51:24293.41
May19 Thu17:50:48293.67
May20 Fri17:50:13293.93
May21 Sat17:49:40294.17
May22 Sun17:49:08294.42
May23 Mon17:48:38294.65
May24 Tue17:48:09294.88
May25 Wed17:47:42295.1
May26 Thu17:47:16295.31
May27 Fri17:46:52295.51
May28 Sat17:46:29295.71
May29 Sun17:46:08295.9
May30 Mon17:45:49296.08
May31 Tue17:45:31296.25
Jun01 Wed17:45:14296.42
Jun02 Thu17:45:00296.58
Jun03 Fri17:44:46296.73
Jun04 Sat17:44:35296.87
Jun05 Sun17:44:25297
Jun06 Mon17:44:16297.13
Jun07 Tue17:44:09297.25
Jun08 Wed17:44:04297.36
Jun09 Thu17:44:00297.46
Jun10 Fri17:43:57297.55
Jun11 Sat17:43:56297.63
Jun12 Sun17:43:57297.71
Jun13 Mon17:43:59297.78
Jun14 Tue17:44:03297.83
Jun15 Wed17:44:08297.88
Jun16 Thu17:44:14297.93
Jun17 Fri17:44:22297.96
Jun18 Sat17:44:31297.98
Jun19 Sun17:44:42298
Jun20 Mon17:44:54298
Jun21 Tue17:45:07298
Jun22 Wed17:45:22297.99
Jun23 Thu17:45:38297.97
Jun24 Fri17:45:55297.94
Jun25 Sat17:46:14297.91
Jun26 Sun17:46:33297.86
Jun27 Mon17:46:54297.81
Jun28 Tue17:47:16297.75
Jun29 Wed17:47:39297.68
Jun30 Thu17:48:03297.6
Jul01 Fri17:48:29297.51
Jul02 Sat17:48:55297.41
Jul03 Sun17:49:22297.31
Jul04 Mon17:49:50297.2
Jul05 Tue17:50:19297.08
Jul06 Wed17:50:49296.95
Jul07 Thu17:51:20296.81
Jul08 Fri17:51:52296.67
Jul09 Sat17:52:24296.51
Jul10 Sun17:52:57296.35
Jul11 Mon17:53:31296.19
Jul12 Tue17:54:06296.01
Jul13 Wed17:54:41295.82
Jul14 Thu17:55:17295.63
Jul15 Fri17:55:53295.43
Jul16 Sat17:56:30295.23
Jul17 Sun17:57:07295.02
Jul18 Mon17:57:45294.79
Jul19 Tue17:58:24294.56
Jul20 Wed17:59:03294.33
Jul21 Thu17:59:42294.09
Jul22 Fri18:00:22293.84
Jul23 Sat18:01:02293.58
Jul24 Sun18:01:42293.32
Jul25 Mon18:02:22293.05
Jul26 Tue18:03:03292.78
Jul27 Wed18:03:44292.5
Jul28 Thu18:04:25292.21
Jul29 Fri18:05:07291.91
Jul30 Sat18:05:48291.61
Jul31 Sun18:06:30291.31
Aug01 Mon18:07:12290.99
Aug02 Tue18:07:54290.68
Aug03 Wed18:08:36290.35
Aug04 Thu18:09:18290.03
Aug05 Fri18:10:00289.69
Aug06 Sat18:10:43289.35
Aug07 Sun18:11:25289.01
Aug08 Mon18:12:07288.66
Aug09 Tue18:12:49288.3
Aug10 Wed18:13:32287.94
Aug11 Thu18:14:14287.58
Aug12 Fri18:14:56287.21
Aug13 Sat18:15:38286.84
Aug14 Sun18:16:20286.46
Aug15 Mon18:17:02286.08
Aug16 Tue18:17:44285.69
Aug17 Wed18:18:26285.3
Aug18 Thu18:19:08284.9
Aug19 Fri18:19:49284.51
Aug20 Sat18:20:31284.1
Aug21 Sun18:21:12283.7
Aug22 Mon18:21:54283.29
Aug23 Tue18:22:35282.88
Aug24 Wed18:23:16282.46
Aug25 Thu18:23:58282.04
Aug26 Fri18:24:39281.62
Aug27 Sat18:25:20281.19
Aug28 Sun18:26:01280.76
Aug29 Mon18:26:42280.33
Aug30 Tue18:27:22279.9
Aug31 Wed18:28:03279.46
Sep01 Thu18:28:44279.02
Sep02 Fri18:29:24278.58
Sep03 Sat18:30:05278.13
Sep04 Sun18:30:46277.69
Sep05 Mon18:31:26277.24
Sep06 Tue18:32:07276.79
Sep07 Wed18:32:47276.34
Sep08 Thu18:33:28275.88
Sep09 Fri18:34:08275.43
Sep10 Sat18:34:49274.97
Sep11 Sun18:35:29274.51
Sep12 Mon18:36:10274.05
Sep13 Tue18:36:50273.59
Sep14 Wed18:37:31273.12
Sep15 Thu18:38:12272.66
Sep16 Fri18:38:53272.19
Sep17 Sat18:39:34271.73
Sep18 Sun18:40:15271.26
Sep19 Mon18:40:56270.79
Sep20 Tue18:41:38270.32
Sep21 Wed18:42:19269.86
Sep22 Thu18:43:01269.39
Sep23 Fri18:43:43268.92
Sep24 Sat18:44:25268.45
Sep25 Sun18:45:07267.98
Sep26 Mon18:45:49267.51
Sep27 Tue18:46:32267.04
Sep28 Wed18:47:15266.57
Sep29 Thu18:47:58266.1
Sep30 Fri18:48:41265.63
Oct01 Sat18:49:25265.16
Oct02 Sun18:50:09264.7
Oct03 Mon18:50:53264.23
Oct04 Tue18:51:38263.76
Oct05 Wed18:52:23263.3
Oct06 Thu18:53:08262.84
Oct07 Fri18:53:53262.38
Oct08 Sat18:54:39261.91
Oct09 Sun18:55:25261.45
Oct10 Mon18:56:12261
Oct11 Tue18:56:59260.54
Oct12 Wed18:57:46260.09
Oct13 Thu18:58:34259.63
Oct14 Fri18:59:22259.18
Oct15 Sat19:00:10258.74
Oct16 Sun19:00:59258.29
Oct17 Mon19:01:48257.85
Oct18 Tue19:02:38257.4
Oct19 Wed19:03:28256.97
Oct20 Thu19:04:18256.53
Oct21 Fri19:05:09256.1
Oct22 Sat19:06:00255.67
Oct23 Sun19:06:52255.24
Oct24 Mon19:07:44254.82
Oct25 Tue19:08:36254.4
Oct26 Wed19:09:29253.99
Oct27 Thu19:10:22253.57
Oct28 Fri19:11:16253.16
Oct29 Sat19:12:10252.76
Oct30 Sun19:13:04252.36
Oct31 Mon19:13:59251.96
Nov01 Tue19:14:54251.57
Nov02 Wed19:15:49251.19
Nov03 Thu19:16:45250.8
Nov04 Fri19:17:40250.43
Nov05 Sat19:18:37250.05
Nov06 Sun19:19:33249.69
Nov07 Mon19:20:30249.32
Nov08 Tue19:21:26248.97
Nov09 Wed19:22:23248.62
Nov10 Thu19:23:21248.27
Nov11 Fri19:24:18247.93
Nov12 Sat19:25:15247.6
Nov13 Sun19:26:13247.27
Nov14 Mon19:27:10246.95
Nov15 Tue19:28:08246.64
Nov16 Wed19:29:06246.33
Nov17 Thu19:30:03246.03
Nov18 Fri19:31:01245.74
Nov19 Sat19:31:58245.45
Nov20 Sun19:32:55245.17
Nov21 Mon19:33:52244.9
Nov22 Tue19:34:49244.63
Nov23 Wed19:35:46244.37
Nov24 Thu19:36:42244.12
Nov25 Fri19:37:38243.88
Nov26 Sat19:38:33243.65
Nov27 Sun19:39:28243.42
Nov28 Mon19:40:22243.21
Nov29 Tue19:41:16243
Nov30 Wed19:42:10242.8
Dec01 Thu19:43:03242.6
Dec02 Fri19:43:55242.42
Dec03 Sat19:44:46242.25
Dec04 Sun19:45:36242.08
Dec05 Mon19:46:26241.93
Dec06 Tue19:47:15241.78
Dec07 Wed19:48:03241.64
Dec08 Thu19:48:50241.51
Dec09 Fri19:49:36241.39
Dec10 Sat19:50:21241.28
Dec11 Sun19:51:04241.18
Dec12 Mon19:51:47241.09
Dec13 Tue19:52:28241.01
Dec14 Wed19:53:08240.94
Dec15 Thu19:53:47240.88
Dec16 Fri19:54:25240.83
Dec17 Sat19:55:01240.79
Dec18 Sun19:55:35240.76
Dec19 Mon19:56:08240.73
Dec20 Tue19:56:40240.72
Dec21 Wed19:57:10240.72
Dec22 Thu19:57:39240.73
Dec23 Fri19:58:06240.75
Dec24 Sat19:58:31240.77
Dec25 Sun19:58:55240.81
Dec26 Mon19:59:17240.86
Dec27 Tue19:59:37240.92
Dec28 Wed19:59:55240.99
Dec29 Thu20:00:12241.06
Dec30 Fri20:00:27241.15
Dec31 Sat20:00:40241.25
 

nothing more to see. please move along.