Skynotes: July 2024
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Time for Aphelion Again
In its elliptical orbit Earth reaches its annual furthest point from the sun this month known as aphelion. It will be 5 million km more distant than its closest point, known as perihelion. which occurred in January. See Earth below for more details.
Melbourne Sun times
| Date | Rise | Set | Day length | Solar noon* |
|---|---|---|---|---|
| Monday 1st | 7:36am | 5:11pm | 9:35hours | 12:24pm |
| Thursday 11th | 7:34am | 5:17pm | 9:42hours | 12:25pm |
| Sunday 21st | 7:29am | 5:24pm | 9:55hours | 12:26pm |
| Wednesday 31st | 7:21am | 5:32pm | 10:11hours | 12:26pm |
*When the sun is at its highest, crossing the meridian or local longitude.
Moon phases
| Phase | Date |
|---|---|
| New Moon | Saturday 6th |
| First Quarter | Sunday 14th |
| Full Moon | Sunday 21st |
| Third Quarter | Sunday 28th |
Moon distances
Lunar perigee (closest to Earth) is on Wednesday 24th at 364,817 km.
Lunar apogee (furthest from Earth) is on Friday 12th at 404,362 km.
Planets
Mercury can be seen around 5:40pm in the north-west at its greatest elongation this month before fading in setting by 7pm. It will appear and set a little later each evening.
Venus is not visible early in the month from Melbourne’s latitude after having passed behind the sun for solar conjunction, but it will become the ‘evening star’ late in the month at dusk from 5:40pm in the north-west.
Earth reaches its furthest distance from the Sun (aphelion) on Friday, July 5th at 152,099,968 km when it will be 5 million km further than its closest (perihelion) which was last January 3rd at 147,100,632 km. Our slightly elliptical orbit principally results from the gravitational influence of our large Moon.
The difference from perihelion to aphelion is noticeable as a 3% change in the Sun’s apparent diameter as observed from Earth, seen clearly in the comparison made by Ian Griffin of Otago Museum in this Astronomy Picture of the Day (APOD) for 9 January 2020.
Our planet’s orbital speed - how fast it takes to orbit the sun - averages 29.78 km/second. But its elliptical path means it moves a little slower at aphelion (29.29 km/sec) and little faster at perihelion (30.29 km/sec).
See NASA Earth Fact Sheet and Notes
Mars continues to be visible rising around 3:20am and moving higher in the north-east before fading in the dawn light by 7am. During the month it will rise a little earlier each morning.
Jupiter passed through solar conjunction but can be observed from 4:45am in the north-east after which it will be lost by 7am in the early dawn light. As the month progresses the jovian giant can be seen a little earlier each morning.
Saturn can be seen this month from 11pm in the east. It will move across the north before fading by 7am in the north-west. By late July Saturn will rise around 10:30pm.
Elongations and Conjunctions
For the inner planets Mercury and Venus, as viewed from Earth, there is an angular separation in our skies between them and the Sun. This is called their Elongation as their position is ‘elongated’ away from the Sun. At Greatest Elongation they appear furthest from the sun.
However, when behind the Sun, there is no elongation and they are at Superior Conjunction ‘conjoined’ with the Sun. When they pass in front of the Sun that is Inferior Conjunction and as daytime objects they are not visible and are again ‘conjoined’ with the Sun.
Find out more at:
EarthSky Astronomy Essentials - Elongation
EarthSky Astronomy Essentials - Conjunction
Meteors
This month we have no strong meteor showers but on the 27th-30th the Southern Delta Aquariids should be visible in the constellation of Aqaurius, the water bearer high in the north. The meteors will radiate from near Skat (Delta Auquarii), the fourth brightest star in the constellation. The best time to look is a few hours before dawn when perhaps twenty meteors per hour could be seen.
See
NASA Southern Delta Aquariids
Earthsky Detla Aquariid Meteor Shower
Stars and constellations
In the west
Canis Major (and Sirius, brightest of the night time stars) is much lower this month in the west.
In the north
High in the north is Virgo and the star Spica, while Leo is in the north-west recognizable by its upside-down hook pattern of stars with the star Regulus. The head of Leo also forms the Sickle, a pattern known to many peoples around the world.
In the east
Scorpius rises much higher in the south-east with the red-giant star Antares easily seen even from areas with city lights. Below and now fully revealed is the centaur-archer Sagittarius whose bow and arrow forms the famous asterism the ‘Tea Pot’.
In the south
The Southern Cross or Crux is high up directly south and, to the left, are the Pointers (Alpha and Beta Centauri which mark the front hooves of the other celestial centaur.
The billions of distant stars and numerous dark dust clouds of the Milky Way forms a broad band across the evening sky from east to west which gradually ‘wheels’ to south to north as the Earth rotates during the night. From our southern perspective we enjoy a superb view of the galaxy quite different to most northern hemisphere locations.
Easy to see in the south-west are the intriguing irregularly shaped neighbouring galaxies, the Large and Small Clouds of Magellan, which are special features of our southern skies.
Visible are the bright stars Canopus in the south-west and Achernar closer to the horizon in the south.
International Space Station
ISS orbits every 90 minutes at an average distance of 400 km appearing like a bright star moving slowly across the night sky. Here are some of the brightest passes expected this month over Melbourne and Central Victoria:
Evening
Wednesday 3rd 6:57pm to 7:01am West-South-West to North
Tuesday 4th 6:09pm to 6:16am South-West to North-East
Morning
Tuesday 16th 6:45am to 6:52am North-West to South-East
Friday 19th 5:59am to 6:02am South-West to South-East
Heavens Above gives predictions for visible passes of space stations and major satellites, live sky views and 3D visualisations. Be sure to first enter your location under ‘Configuration’.
On this day
1st 1770, closest pass to Earth of any known comet: Comet Lexell at 2.2 million km (roughly 5.5 times the moon’s distance).
4th 1054, Chinese and other astronomers witness the supernova explosion that produced the Crab Nebula 6500 light years away.
4th 2005, Deep Impact probe (USA) crashes into Comet Tempel 1 to analyse its composition.
4th 1868, birth of Henrietta Swan Leavitt who established the luminosity-period relationship of Cepheid variable stars allowing Edwin Hubble to show ‘nebulae’ were other galaxies beyond our own.
5th 1687, Isaac Newton’s Mathematical Principles of Natural Philosophy is published laying the groundwork for much of modern science.
7th 1959, Venus’ diameter is determined and its atmosphere analysed by its occultation of the star Regulus in Leo.
8th 2011, the space shuttle Atlantis (USA) is launched on the final mission for the shuttle program.
10th 1962, first communications satellite Telstar (USA) is launched as an experiment in trans-Atlantic communication.
11th 1979, Skylab 1 (USA) is destroyed during re-entry over Western Australia and scatters debris over a wide area.
14th 1965, Mariner 4 (USA) makes the first controlled flyby of Mars and returned the first close-up images of the planet.
15th 1975, Apollo-Soyuz Test Project mission occurs. An American Apollo module with three astronauts (launched with the last Saturn rocket) and a Soviet Soyuz craft with two cosmonauts rendezvous and dock in Earth orbit.
16th 1746, birth of Giuseppe Piazzi, discoverer of the dwarf planet Ceres in the asteroid belt.
16th 1969, Apollo 11 (USA) launches to moon for the first lunar landing 5 days later.
16th 1994, Jupiter is struck by over twenty fragments of Comet Shoemaker-Levy 9 with Earth-based telescopes and the Hubble Space Telescope observing the unique event.
20th 1969, Apollo 11 (USA) Moon landing when Lunar Excursion Module ‘Eagle’ touched down in the Sea of Tranquility.
21st 1969, first humans set foot on another world. At 12:39pm AEST Neil Armstrong becomes the first person to step onto the Moon and is followed 20 minutes later by Buzz Aldrin, while the third Apollo 11 astronaut, Michael Collins, remains in the Command Service Module ‘Columbia’ in lunar orbit. Armstrong and Aldrin are on the moon for 21hrs 36min.
22nd 1784, first use of parallax and hence calculation of the distance to a star by Friedrich Bessel.
23rd 1995, Alan Hale and Thomas Bopp discover comet now named after them.
23rd 1928, Vera Rubin born – famous for analysing rotation rates of galaxies.
24th 1969, first lunar landing mission, Apollo 11, ends with Command Module splashdown in the Pacific Ocean.
30th 1971, first Lunar Rover used on the moon in Apollo 15 Mission.
29th 2005, dwarf planets, Eris and Makemake, are announced leading to Pluto becoming a dwarf planet as well.