Why Doesn't the Moon Fall Down?

Public domain image via Pixabay.

 

 

It's a question a child or for that matter a curious adult might ask. And what about the International Space Station or ISS, something the size of a football pitch? How can that stay aloft? The fact is that both the Moon, the ISS and any other object in orbit are actually falling. However just like someone chasing a rainbow to get to the proverbial crock of gold, the Moon never reaches the ground.

 

Launching Projectiles

 

Trajectory of a projectile fired horizontally. © Eugene Brennan

 

Imagine you throw a ball or stone horizontally out a window or a cannon on top of a tall building fires a cannon ball. Once the ball leaves the muzzle of the gun, it doesn’t travel any faster horizontally. In fact it slows down due to air resistance, known as drag. However, we’ll neglect this and imagine the cannonball is travelling through a vacuum. As it’s moving outwards horizontally, it’s also falling because of gravity. The combined horizontal and vertical motion cause the cannonball to follow a curved path. In fact the trajectory is a shape called a parabola. The higher the initial speed of the cannonball, the further it’ll travel outwards before it hits the ground.

 

What if the ground isn’t flat, but curved?

 

If the ground is flat, the projectile will eventually hit the ground. But imagine if it’s fired at a much higher velocity so it goes over the horizon. This time it’ll take longer to hit the ground because the ground drops off below it due to the curvature of the Earth. As the initial velocity is made higher and higher, the cannonball keeps travelling further and further horizontally but continues to fall downwards. However the ground keeps dropping below it because of the continuing curvature of the Earth. So it's moving away from the Earth because of the curvature, but at the same time falling and getting closer. The two motions cancel each other out and eventually the projectile will circle the Earth until it gets to its starting point. It will then do this indefinitely without further assistance from an engine or other means to propel it. We say that it has reached orbital velocity. Also the spiralling trajectory becomes a near circle. For low Earth orbit of from 200-2000 km altitude, orbital velocity needs to be 7.7–6.9 km/s (27,772–24,840 km/h or 17,224–15,435 mph). This is why such huge rockets are needed to launch satellites into space. Engines have to accelerate satellites to 10 times the speed of an AK-47 assault rifle bullet. Orbital velocity also decreases with distance so the Moon orbits the Earth at a mere 3683 km/h or about four and a half times the cruising speed of a jetliner.

 

Image courtesy Brian Brondel, CC by SA 3.0 via Wikipedia

https://creativecommons.org/licenses/by-sa/3.0/

 

Escape Velocity

If an object's velocity is increased sufficiently, it can break free of Earth's gravity. The escape velocity at ground level on the Earth's surface is 11.19 km/s (6.95 miles/s) or 25,031 mph. Spacecraft travelling to the Moon or to other planets must reach this speed so they can travel on an outwards trajectory.

Air resistance known as drag does slow down satellites somewhat because space isn't a perfect vacuum and some gas molecules exist at orbital altitudes. This causes satellites to spiral closer to the Earth and they sometimes need to be boosted back to their original orbits. According to New Scientist, 7.5 tonnes of fuel are used each year to maintain the ISS at its orbital altitude. In theory a satellite could operate close to ground level but once the thrust from a rocket engine is turned off, it would soon slow down and spiral downwards towards Earth due to drag. Spy satellites are sometimes used in low Earth orbit, but have a limited lifespan due to this drag.

Rainbows Don't Have Seven Colours

AI image created on request using Bing Image Creator

 

"Richard of York Gave Battle in Vain" or ROYGBIV was the mnemonic we used in school to remember the seven colours of the rainbow: Red, orange, yellow, green, blue, indigo and violet. But did you know there aren't actually seven colours? Isaac Newton, the 17th century scientist decided to add another colour because "7" was a perfect and divine number (God made the world in seven days, there were seven known objects in the Solar System, seven musical notes on a scale etc). So he added the seventh colour, indigo, whereas there are actually only six. Rainbows are composed of a continuous variation of spectral wavelengths from short to long as white light is bent or refracted, and "smeared" or dispersed by differing angles depending on wavelength as it passes from air through a different medium such as glass or water. The apparent colours are in the eye of the beholder simply because we have cones for red, blue and green in our eyes. Mantis shrimps have many more than three types of cones in their eyes than us, and would see more than six colours in a rainbow.

AI image created on request using Bing Image Creator

BPS Podcast - Not Just a Phage

Image courtesy Guido4. CC BY-SA 4.0 via Wikimedia Commons

We don't really know what life is. The best scientists can do is describe the attributes of living organisms, the most obvious and superficial being that they seem to have purpose in their actions, they eat or absorb nutrients from their environment and they reproduce.
Are advanced AI systems or even your phone "alive"?
Robots of the future will probably be incredibly complex and already some robots are autonomous meaning they can be given a general task and can go off and work out how to do that themselves. The Perseverance rover on Mars is semi-autonomous and has purpose, its task being to search for evidence of past microbial life. While not reproducing biologically, future robots may be capable of replicating themselves using raw materials they source. But does that make them alive? Single celled organisms like bacteria "feed" by absorbing nutrients through their outer walls or cell membranes. Bacteria are thought to be living because they satisfy the criteria for life. However the jury's out for viruses as they can't do anything useful on their own and simply consist of virus particles called virions, made from strands of RNA or DNA. To reproduce, they must infect a host cell, causing the machinery of the cell to make thousands of copies of the virus which then emerge to infect more cells. Bacteriophages or simply phages are types of viruses that infect bacteria and archaea (single celled organisms without a nucleus). Treatment of diseases with phages that could destroy infectious bacteria was a practice once used in the early part of the 20th century but the development of new antibiotics over the decades meant that phage treatment was sidelined. However the appearance of antibiotic-resistant bacteria (something predicted by Alexander Fleming who discovered penicillin in 1928) in the late 20th century and the slowdown of discovery of new antibiotics prompted the WHO in 2017 to highlight the need for new approaches to treating disease.
In this Big Picture Science podcast from the SETI Institute, the team investigate bacteriophage treatment to combat the antibiotic crisis.

Dutch Billys and Drains

Dutch Billy in the slums of Dublin. Photographer Robert French. Image courtesy The National Library of Ireland, Lawrence Photograph Collection.

Straying a bit again from the scope of this group into my other interest, architecture. Not that I know a huge amount about it, but as a child I always had a fascination with and appreciation of the design of buildings and structures new and old. This is one of several drawings of Kilcullen Bridge made over the last 250 years or so. It's from a set of four illustrations of Kildare held in the Manuscript & Archives Research Library of Trinity College Dublin, drawn in 1795 by Sir William Smith, an artist and captain in the Royal Engineers. You may have seen the illustration before, but the image in the TCD archive  is a higher resolution version. What's interesting is the terrace of buildings with street-facing gables and tall chimneys, approximately located in what is now the square. Could these be Dutch Billys, or was it just artistic licence and an embellishment of the drawing by Smith? So called Dutch Billys, reputedly named after King William of Orange, were a style of pre-Georgian architecture, brought to Ireland by French Huguenots and Dutch and Flemish protestants fleeing persecution in the late 17th century. The style is common in Amsterdam. Many of the Billys in Dublin and other towns were later either "Georgianified" by having walls built in front of their gables or demolished in the last century as they fell into disrepair, which was a shame. Maybe the three story building adjacent to the lane down to Brennan's yard was the remains of this terrace? This was demolished in the early 70s to make way for the building The River Cafe is located in. It didn't have a gable, but the roof could have been modified. The other thing I noticed are the arches at river level. What could they have been for? Were they drains to allow water to flow back to the river from the square when water level dropped? Before the dam was built on the Liffey in the 1940s and regulated water flow, flood waters extended into the square as far as Brennan's Hardware and on one occasion, according to my late neighbour Fred Maher, the bridge in Athgarvan was closed for fear of it being washed away when the water level became dangerously high. There appears to be the gable wall of a building behind the two arches with what could be a mullioned window (the division between two openings visible in the higher resolution image). A medieval building which was gone by the time the first edition OSI map was drafted? On this map, there's also a structure in the square, in front of what is now McTernans. Could it have been a market house? We'll probably never know.

References:

Wexford archaeologist Colm Moriarty's article about Dutch Billys.

 

Kilcullen Bridge illustration in the TCD archives. This is a higher resolution version of the image on the Kildare Archaeological Society's website.

 

Con Costello's article about Sir William Smith's four illustrations of Kildare.

The Leinster Aqueduct and Kilcullen Mills

More exploration on this week's Sunday cycle trip along the canal.

 

Flood arch under Leinster Aqueduct. © Eugene Brennan

 

I was going to go as far as Digby Bridge, but it was 6.30 on a really gloomy day and I wanted to get back to Kilcullen before it got too dark, so I'm going to have a better look at this structure at a later date. This is the underside of the Leinster Aqueduct, constructed in 1783, which carries the canal over the River Liffey. There seem to be several of what look like flood arches at this section, quite high up on the river bank away from the main arches. The New Bridge at Carnalway (as it's described on the c. 1837 OSI first edition map, because there's an older 17th century bridge upstream on the Harristown estate) also has one or more flood arches (I think) on the northern side of the bridge, but one of these is described as a tunnel on the later 25" c. 1900 map. This may have been an access route to the corn mill shown on the first edition map. The corn mill and mill pond seem to have disappeared by the time the 25" map was created.

 

New bridge over the River Liffey at Carnalway. shown on the c. 1837 first edition 6" OSI amp. Map image courtesy OSI (Tailte Éireann)

 

Bridge over River Liffey at Carnalway. The corn mill has disappeared by the time the 25" map was drafted. Map image courtesy OSI (Tailte Éireann)

 

19th Century Mills in Kilcullen

On the subject of mills, in addition to the mill at New Abbey and in the centre of the town, there was also one located in the fields between The Stray Inn/The Mill pub and the Green Avenue, also with a small mill pond to store water (the 19th century equivalent of a battery storage facility). This was fed by a mill race off the Mill Stream, splitting from the stream inside the grounds of Gilltown estate. Strangely, the New Abbey mill has no mill pond shown on the map, possibly because it no longer existed by the time the 1837 map was drafted. The area around the Mill Stream south of the road bridge is quite flat, so perhaps that acted as a natural reservoir to hold water, with flow controlled by a sluice further downstream. Alternatively If flow was sufficient and constant on the stream, maybe a mill pond wasn't necessary? The feed to this mill passed through an arched culvert under the road, a short distance south of the bend at the entrance to the cemetery. The mill was located just east of the wall around the cemetery, the ruin in the field possibly being part of the structure. There was actually what looked like a small small weir (although it may just have been a change in the level of the stream bed), visible on the south side of the bridge in the 70s. From what I can remember this was only around a foot tall (see photo from 1983 below), so possibly it was the remains of a taller structure. There was also a saw mill on the Gilltown estate and the current lake was possibly the mill pond that supplied water to this. Maybe someone knows the history of this? It's shown on the c. 1900 map.

 

The Mill Stream, Kilcullen, south of the bridge on the New Abbey Road. The photo was taken in 1983. © Eugene Brennan

 

What were mill ponds for?

A mill pond acts like a buffer, analogous to the tank on an air compressor, the latter storing energy so that it can be released in large dollops when necessary, greater than a compressor pump could deliver on it's own. Anyone who worked in Renley Engineering may remember the flywheel on a punching machine, which had the same function. The advantage of a mill pond is that it can store water from a stream with a relatively low flow rate, even during the night, and a sluice gate can be opened to release a larger flow when required, greater than a stream itself could source. A weir has a similar function, backing water up behind a barrier, often on a river. A weir however raises water level so that the water gains potential energy because of its height. Some mills don't seem to have had weirs. Instead, a mill race carried water from a point further and often a long distance upstream where the water level was at a higher elevation.

You can view old maps on the OSI's Irish Townland and Historical Map Viewer here:

https://osi.maps.arcgis.com/apps/webappviewer/index.html...

Map images courtesy OSI (Tailte Éireann)

Rust Cleaning Laser

Costing around half a million dollars in 2017, maybe someday we'll see them in the centre aisle of Lidl. It uses a process called laser ablation or photoablation, where energy from a laser turns rust into gas by sublimation (I.e. it doesn't pass through the intermediate liquid phase., unlike boiling ice to form steam where the ice melts to water first). At higher laser flux, rust is converted to plasma, the fourth state of matter (solid, liquid and gas being the other three).

Long COVID

Graphic by Ian Dennis - http://www.iandennisgraphics.com. File is licensed under the Creative Commons Attribution 4.0 International license.

I totally lost my sense of taste and smell in early 2020 when there were only a few known COVID-19 cases in Ireland and loss of taste wasn't one of the recognized symptoms. It was before home test kits were readily available and I never had a test that confirmed I had COVID, but the general belief is that the disease had been circulating in Ireland from the previous autumn. I had no other symptoms and slowly regained my taste and smell over 10 days or so, but it never fully recovered and I still can't properly smell or taste many things, one of which is chocolate. That's understandable since chocolate and coffee are a complex cocktail of hundreds of chemical compounds and I probably lost sensitivity to lots of those. In this article in The Conversation, Ziyad Al-Aly, Chief of Research and Development, VA St. Louis Health Care System and Clinical Epidemiologist, Washington University in St. Louis writes about long COVID and how it affects the body.