Today the northern hemisphere of the Earth is maximally inclined away from the Sun, producing the shortest day. This is due to the axial tilt of the Earth, driving the seasons as we hurtle around our home star each year.
From here on, imperceptibly at first, our days grow longer in the northern hemisphere and shorter in the southern hemisphere.
This change in daylight is like a trigonometric Sine wave and will accelerate as winter advances, reaching its greatest rate of change near the Spring equinox in March.
The image I’ve shared was taken from inside one of the the passage cairns at Clava a few years ago on Dec 21st, a site with claimed mid winter significance. Sure enough light flooded into the back of the cairn via the south western aligned passage.
The truth is we don’t really know the real significance of these structures, and are left to speculate, sometimes more wildly than the evidence deserves. But it’s fun and captivating to imagine what could well have been ancient connections linking landscape, culture and the heavens above.
The NASA Hubble space telescope has imaged the most distant star ever detected at a staggering 12.9 billion light years away. The light captured from Earendel (dubbed the ‘Morning Star’) is a snapshot from an epoch when the universe was only 1 billion years old, making it significantly older than the previous furthest star detected by Hubble in 2018. (That star was dated to 4 billion year after the big bang).
Normally stars at such immense distances would be undetectable, but its discovery was aided by the gravitational distortion from distant galaxy clusters, magnifying the star and its host galaxy in a phenomena called ‘gravitational lensing.
Gravitational lensing is analogous to the refraction of light from a glass lens, magnifying and revealing objects that would normally be occulted by closer structures by the bending of space near areas of high mass – like galaxy clusters. Sometimes duplicate images of the same object can be seen, creating copies of the object along symmetrical arcs. The image below illustrates this effect on a star cluster which appears either side of Earendel.
You might wonder how immense distances like this can be calculated given the complexity and uncertainty in pin pointing the distance to relatively close stars, let alone objects billions of light years away?
The principle tool used to measure these vast distances is an object’s spectral redshift – a measure of how much its light rays have been stretched (made longer) due to the fabric of space itself being stretched the further away we observe. Larger redshifts indicate objects that are further away – a relationship first accurately established by Edwin Hubble when cataloging the spectra from many distant galaxies.
Given the redshift of an object we can calculate its recessional speed (related to the global expansion of the universe) and from this its distance can be determined using the Hubble’s constant Ho. These calculations can be set out very simply:
V (recessional speed) = Red-shift x Speed of Light
In the case of Earendel the detected redshift from its spectra was 6.2. Therefore:
V (Earendel) = 6.2 x 300 million m/s = 1860 million m/s.
It’s important to note that this speed is faster than the speed of light! How can this be? Well this is actually a measure of the speed that space itself is expanding. Light cannot travel faster than 300 million m/s – our cosmological speed limit – but there is no limit on the rate at which the fabric of space can expand. In fact for general relativity to work space must be permitted to expand at potentially unlimited rates.
From the recessional speed we then use Hubble’s law to find the distance to the star:
D (distance) = V (recessional velocity) / H0 (Hubble’s constant)
This gives our published distance to Earendel of 12.9 billion light years! A staggering distance taking us back to the earliest period of star formation when the abundance of atomic elements in the universe was very different to today.
We believe the very first population of stars emerged around 100 to 250 million years after the big bang, so Earendel formed only a few hundred million yeas after this. The new James Webb telescope will likely continue to study Earendel in the infrared, at longer wavelengths, potentially revealing the star’s temperature and luminosity and therefore its stellar classification.
I wanted to share some images with you that had me transfixed when I was a young boy (and still do to this day). I recall first seeing them in a hardback book of my father’s called Cosmos (which presumably accompanied the TV series that was being broadcast at the time).
The images depict the fate of our planet as the Sun transitions into a red giant star, at the very end of its life, some 4-5 billion years from now.
As the temperature of the Sun slowly increases, the oceans recede and our precious atmosphere is stripped away. Eventually the whole horizon is overwhelmed by the Sun in a bloated distended form, with the final image showing the Earth completely barren and parched.
I remember wondering at the time – where would all the people and animals be? Would we perish or find some new star to call our home? I think it was the first moment I glimpsed the immensity of stellar time scales and how tiny human lives and endeavours appeared to be next to these vast physical processes.
This is still what fascinates me most about astronomy and cosmology, and it’s amazing how something as natural and simple as looking up at the stars is a gateway into these incredible realms of the imagination.
Anyway here are the images, including their original captions. I was also pleased to find out that Adolf Shaller is still producing amazing art. Try an image search on Google with his name and enjoy.
Changing position of Sunrise from a fixed location over a year
The changing position of Sunrise throughout a year from a fixed location. The further north or south of the equator we live the more extreme our seasonal changes and the bigger shifts we perceive in the sunrise or sunset position during the year. In such harsh and changing seasons it would also have been the more important for ancient cultures to mark the seasons.
Using the landscape to mark the seasons like this is called a horizon calendar. But what if your horizons are flat and featureless, or you require more accuracy, or you’re a powerful priest and wish to theatricise important changes in time?
Then ‘perhaps’ you construct an artificial horizon by placing large stones to mark the progress of the Sun – a henge.
In view of recent developments my contribution to this years Inverness Science Festival will be a free live streamed talk. Please visit my Highland Astronomy facebook page for more details:
Astronomer Stephen Mackintosh will take you back in time to discover how our distant ancestors used the Sun, Moon and stars to track the progress of time and the seasons. Looking at ancient monuments connected to the night sky, we’ll go on a tour of Egypt, Central America, southern England and back home to Scotland where some of the finest concentrations of neolithic structures exist anywhere in Europe, not least the wonderful Clava Cairns. Plus advice on sky watching and naked eye observing you can put into practice yourself.
Note: this event is free and will be live streamed online as part of the Inverness Science Festival’s adjusted programme.
The main henge posts are now in place. Markers and smaller posts for the Celtic cross quarter days still to be added.
More progress on the wood henge and Celtic calendar up at Abriachan Forest, with the main posts for the meridian, equinoxes and solstice rise and set positions now in place.
Not quite finished yet. Abriachan plan to sow seeds for next few weeks and let the ground within the perimeter settle.
Descriptive marks for the main posts and small posts for the Celtic Cross Quarter days Imbolc, Lammas, Samhain and Beltane will be added later.
In addition to tracking the Sun and measuring the solar year, we can use the henge during the stargazing programme to record the rising of new seasonal constellations in the East and rough measurements of the transit altitudes (due south) and azimuth positions of the stars.
We also had a great kick off to the Star Stories program last night with ancient astronomy learning, storytelling and activities for the young ones.
The next event will be Nov 23rd with guest astronomer and author Steve Owens (aka Dark Sky Man). Booking links will go up shortly.