Royal Astronomical Society

• Six months later a smaller crowd congregates around the Heel stone to witness the midwinter Sun setting within the stone circle.
• But a hypothesis has been around for 60 years that part of Stonehenge also aligns with moonrise and moonset at what is called a major lunar standstill.
• There is now an abundance of archaeological evidence that indicates the solar alignment was part of the architectural design of Stonehenge.

Lunar standstill

• It is these longer sides that are thought to align with the major lunar standstill.
• These southern and northern limits of moonrise (or set) change on a cycle of 18.6 years between a minimum and a maximum range – the so-called minor and major lunar standstills, respectively.
• The major lunar standstill is a period of about one and a half to two years when the northernmost and southernmost moonrises (or sets) are furthest apart.

• The strongest evidence we have for people marking the major lunar standstill comes from the US southwest.
• Of six cutting dates, four correspond to major lunar standstill years between the years AD1018 and AD1093, indicating that the site was renewed, maintained or expanded on consecutive major standstills.
• Returning to southern England, archaeologists think there is a connection between the major lunar standstill and the earliest construction phase of Stonehenge (3000-2500 BC), before the sarsen stones were brought in.
• The major lunar standstill hypothesis, however, raises more questions than it answers.

A search for answers

• It’s unclear whether the Moon would have been strong enough to cast shadows and how they would have interacted with the other stones.
• This collaboration will result in events showcasing and debating the lunar alignments at both Stonehenge and at Chimney Rock.

Erica Ellingson receives funding from the US Department of Agriculture. This institution is an equal opportunity provider. Amanda Chadburn and Fabio Silva do not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and have disclosed no relevant affiliations beyond their academic appointment.

• But now our new study, published in Monthly Notices of the Royal Astronomical Society, has revealed that these two ice giants are actually very similar shades of greenish blue.
• These were recorded in 2002 (Uranus) and 2003 (Neptune).
• We found that the colours of Uranus and Neptune were actually remarkably similar, with Neptune appearing only slightly bluer – see the image below.

Reconstructing the colours

• Our reconstructed colours of Uranus and Neptune look very different from previous images, which come from the Voyager 2 spacecraft’s encounters with these planets in 1986 and 1989 respectively.
• So, did the colours of Uranus and Neptune change between the late 1980s and early 2000s?
• Both of these instruments record images where each individual pixel is a complete, continuous spectrum covering all colours that can be seen with the human eye – making them more accurate than spacecraft when it comes to colour.
• This allowed us to determine unambiguously the actual colour that the human eye would perceive for Uranus and Neptune.

Uranus changes colour

Comparing the true colour of Uranus in 1986 with more recent observations, it became clear that Uranus in 1986 was actually slightly greener than it was in the early 2000s. We tried to determine why this was the case by turning to observations made between 1950 and 2016 at the Lowell Observatory in Arizona. These observations contained the overall brightness of Uranus and Neptune almost annually at two wavelengths: green and blue.

• This revealed that Uranus does change colour, becoming greener at the solstices (when the Sun’s path in the sky is the farthest north or south from the planet’s equator) than it is at the equinoxes (when the Sun’s path crosses the planet’s equator).
• Part of the reason for this colour change is that Uranus spins almost on its side during its 84-year orbit about the Sun.
• This modified model then substantially reproduced the Lowell observations and thus explains how the overall colour of Uranus changes during its orbit about the Sun.

Patrick Gerard Joseph Irwin has received funding related to this study from the UK Science and Technology Facilities Council.