Skynotes: July 2025
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It’s Aphelion Time
In its elliptical orbit Earth reaches its annual furthest point from the sun – aphelion – this month on Friday, 4th July. On that date it will be 5 million km more distant than its closest point – perihelion - which occurred in January. Our planet’s slightly elliptical orbit principally results from the gravitational influence of our large Moon.
And while Earth may be slightly closer to the Sun at aphelion, and a little further at perihelion, the variation in solar radiation we receive is negligible compared to the effect of Earth’s tilted axis that drives the seasonal extremes of winter or summer.
The difference from perihelion to aphelion is only a 3% change in the Sun’s apparent diameter as observed from Earth, seen clearly in this composite by Ian Griffin in this Astronomy Picture of the Day (APOD) for 9 January 2020.
Our planet’s orbital speed, how fast it moves, varies too. It 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). When spacecraft are launched to other bodies in the solar system, or if a gravity assist ‘sling shot’ by Earth, or a sample return mission is planned, then Earth’s varying orbital speed must be accounted for.
Melbourne Sun times
| Date | Rise | Set | Day length | Solar noon* |
|---|---|---|---|---|
| Tuesday 1st | 7:36am | 5:11pm | 9:35hours | 12:24pm |
| Friday 11th | 7:34am | 5:17pm | 9:42hours | 12:25pm |
| Monday 21st | 7:29am | 5:24pm | 9:55hours | 12:26pm |
| Thursday 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 |
|---|---|
| First Quarter | Thursday 3rd |
| Full Moon | Friday 11th |
| Third Quarter | Friday 18th |
| New Moon | Friday 25th |
Moon distances
Lunar perigee (closest to Earth) is on Sunday 20th at 368,041 km.
Lunar apogee (furthest from Earth) is on Saturday 5th at 404,627 km.
Planets
Mercury is an evening planet seen from 5:50pm at disk in the north-west before setting around 7:15pm, but by mid-month it will be too close to Sun as it moves to Inferior Solar Conjunction passing in front of the Sun.
Venus continues to be seen as the ‘morning star’ from 4:20am in the east and then north-east before being lost to the dawn light by 7:15am. It will appear a little earlier each morning during the month.
Mars remains visible from 6pm in the north-west at twilight but will set by 9:40pm, but will be seen a little earlier each evening until by late July from as early as 6pm.
Jupiter is too close to the sun for most of this month as it is has passed behind the Sun for its Solar Conjunction, however, by the end of July it will appear again in the early dawn sky from 5:45am in the north-east.
Saturn remains a mostly morning object in the night time skies rising from midnight in the east, moving across the north and fading by 7am at dawn in the north-west. It will appear a little earlier each night so that by the end of the month it will rise from around 11:30pm.
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’. Through the constellation of Sagittarius lies the centre of the galaxy and its supermassive black hole known as Sagittarius A* imaged in 2019 by the international collaboration Event Horizon Telescope (EHT) with results released in 2022.
See
Astronomy Australia: First direct evidence of supermassive black hole
ESO: EHT Meet the black hole at the centre of our galaxy
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
Tuesday 1st 6:51pm to 6:54pm, South-West to East
Wednesday 2nd 6:02pm to 6:09pm, South-West to North-East
Morning
Wednesday 16th 6:34am to 6:44am, West-North-West to South-East
Thursday 17th 5:48am to 5:52am, North-North-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.