Astroblog: Adelaide Henge 2019

Tuesday, March 19, 2019

Adelaide Henge 2019


Rundle Mall looking west	Hindeley street looking west

The equinox is on the 21st, where the Sun rises due east and sets due west and day and night are equal length. Aside from the marking of the passing of summer to Autumn, it is also a chance to see "city henge", in this case Adelaide henge.

Unlike the famous Stonehenge, whose stones are aligned to catch the summer and winter solstices. City henges are found in cities with a regular grid of east-west streets, where the rising and setting sun can shine through these human-made canyons at various times of the year. The most famous is Manhattan henge, where the skyscrapers of New York provide dramatic chasms for the setting sun to illuminate.

Less well known but still dramatic is Melbourne Henge. Where great views are to be had down Collins Street and Bourke street amongst others as the setting sun washes the streets in golden glow.

And the phenomenon can be seen in Adelaide too. We lack the skyscraper canyons, but our CBD east-west streets can still be illuminated with a golden glow.

Did I say east-west? while our CBD streets are nominally east west, they are infcat slightly out of alignment with true east (the streets of Melbourne are even more out of true being angled at 250 degrees west, which is why Melbourne Henge for the setting sun is 7 February and 4 November, not on the days of the equinox).

As our street are closer to East-west (North terrace being 87 degrees east and 263 degrees west as measured by my trusty compass) Adelaide henge is closer to the equinox. Sadly our best time to observe the sun setting through the city has passed. This was on the 16th at 19:33, when the sun was setting at 263 degrees. However, the sun moves slowly from this ideal alignment and our streets are broad, so good sunsets almost aligned with the streets such as North terrace, Rundle Mall and Hindle street should be available over the next few days.

In contrast, best theoretical sunrise is not until 28 March, when the sun is 87 degrees from East and rising at 7:25. However, to the east is the Adelaide hills, and by the time the sun rises above then it will have moved off the direct line. Ironically the best time for the sunrise to shine down the street is on the 21st, at 7:36, as the sun just clears the Adelaide hills.

Again, those are the best times, but a few days either side of those it will still be good. So get up early or stay in the city for sunset, and you might see something wonderful.

Labels: equinox, public outreach., Sun