C/2023 A3 Tsuchinshan-ATLAS

Comet Tsuchinshan-ATLAS was one of the brightest comets of the last century. Its high inclination orbit made its apparition at peak brightness relatively short-lived but quickly gave it separation from the Sun, making it relatively easy naked-eye viewing in the evening sky. The ion and gas tail were not very independently resolvable due to the comet's outgoing trajectory being nearly in a co-radial from the sun, but nonetheless the tail as quite long, visually appreciable, and accompanied by an anti-tail.

I took some photographs of Tsuchinshan-ATLAS in Bortle 5 suburban conditions on the evening of October 15. Stellarium estimates the comet was a +1.4 magnitude moving through the constellation Serpens. Observations listed on cobs.si suggested magnitudes in the range of +1 to +2.

The comet in the early twilight sky. 5×2' stack. 

Closer shot of the central region. The anti-tail was slightly visible from this suburban location, but didn't come across as clearly in these images. Another comet, 13P/Olbers, is technically in this shot, but as a dim +10 magnitude comet it was not particularly visible. 5×2' stack. 

More detail of the sky near the coma. The anti-tail's a bit more prominent here, as is the globular cluster Messier 5, also known as the Rose Cluster, to the right of the coma. 10×2' stack.  

 

Detail of the faint anti-tail region. 10×2' stack. 

 

 

Heavily stretched/edited 10×2' stack seeking to bring out finer details in the anti-tail. Was also hoping to pull out 13P/Olbers, but it wasn't really evident in any of the photographs I took.

 

Merger of two separate 10×2' stacks, with one focused on the coma/anti-tail and the other focused on the tail. 

 

That was definitely at least a 12 degree tail. 20×2' stack.

Dayside to nightside wind

I recently thought of something that could be a fun introductory exercise, encapsulated in the following scenario:

After learning in her ATM 101 class that many of the large-scale flows in the atmosphere are set in motion by temperature differences, a student wonders whether that means there is a wind that constantly blows from Earth's daytime to nighttime side.

Let's suppose we're at the equator, ignoring Coriolis effects, and ignore differences in terrain and surface type. For simplicity, let's take an extreme and approximate the "wind" that should be produced between the hottest and coldest places along the equator (on land), assuming that these places are exactly antipodal. Maybe the warm place is in late afternoon and the cold place is in the early pre-dawn morning.

Given we're at low latitudes, let's invoke the weak temperature gradient approximation for the tropics, which asserts that temperatures are horizontally uniform in the free troposphere due to large Rossby radii of deformation, enabling rapid homogenization of the thermal field by gravity waves.

A 700 mb height of 3150 m seems pretty reasonable for the equator. Let's say both places have that exact 700 mb geopotential height and a 700 mb temperature of 280K, but the sultry afternoon locale has a surface temperature of 313K while the cold morning locale has a surface temperature of 292K.

Invoking the hypsometric equation, and ignoring moisture, the implied surface pressure (at 0 m) of the hot location is around 1006 mb, while the cold location the implied surface pressure is around 1020 mb. Let's assume the density of air to be not significantly different, at 1.225 kg/m3. As the locations are antipodal, let's say that as the equatorial radius of Earth is about 6,378 km, the locations are separated by 20,000 km, or half of Earth's circumference. Then, the pressure gradient is about 0.0007 Pa/m, which gives an acceleration of around 0.00006 m/s². 

This acceleration, on the order of 10^-5 m/s², is incredibly tiny, considering that synoptic horizontal flows tend to be on the order of 10 m/s, and even a very mild synoptic-scale pressure gradient of 100 Pa over 1000 km produces a greater pressure gradient force. One can imagine that local differences in heating and terrain are more influential to local winds, and the solar terminator moves far too fast (around 450 m/s) to really allow this sunlit-to-unlit wind to foment without perturbation. 

The case may be different outside of Earth. For example, on Venus, which rotates once every 243 Earth days, there is a fast westerly Venusian wind aloft that transports heat from the dayside to the nightside.