Transient Targets

I'm casting a fairly wide net in this article. I'm including things outside the solar system, which are typically described as Time Domain Astronomy, but I'm also going to include things inside the solar system. Basically, anything changeable, unpredictable, and/or unusual.

Over sufficiently long timeframes, things we think of as constant have changed. For example, the stars through the galaxy, so the arrangement of stars we currently see will change over tens of thousands of years.

Fun fact: the star Polaris is near the north celestial pole, but because earth precesses like a spinning top, the north celestial pole shifts over time.

Earth

Let's start with earth.

Aurora

Aurora are caused when oxygen and nitrogen molecules in the upper atmosphere get ionized due to plasma from the sun getting directed towards earth's polar regions. (Typically.) This is due to earth's magnetic field. If the solar storm that sent the plasma to earth is strong enough, the aurora can be visible at much lower latitudes. (Which is what happened on and around May 10th, 2024.)

Meteors and meteor showers

Meteors are caused by small pieces of rock or other such things hitting the upper atmosphere at high speed and vaporizing. Meteor shows are related meteors caused by a bunch of space debris traveling in the same orbit around the sun and intersecting earth. Some meteor showers have been traced to comets, and the debris they leave in their orbit as they circle the sun.

These are easily visible in a dark sky. Most meteor showers are just a few meteors, or perhaps a few dozen meteors, per hour. Every once in a while, there will be a particular bright meteor (a fireball). And very rarely you'll see one explode (a bolide).

Satellites

I'm not going to discuss the numerous satellites that orbit earth except for a few exceptions.

Satellite flares

Satellite solar panels can reflect sunlight, and if they're at the right angle, the reflection is visible from the ground. If the solar panels are large enough, it can be quite bright.

One of the first examples of this was with the Iridium constellation of satellites in the 1990s. At times, the flares could be visible during the day.

See: Satellite flares

Satellite transits

Transit

When a visually smaller object crosses a visually larger object in the sky. See also: Astronomical Transit

Sometimes satellites will transit the moon or the sun. Satellites from earth's surface appear quite small, but particularly large things, like the International Space Station, are doable and can be photographed with a small telescope like a Seestar.

I use the Transit Finder website to check for ISS transits near me. I used it to capture the ISS crossing the sun from a nearby park.

Solar system

There are no natural moon transits. Well, unless a near earth asteroid comes between earth and moon.

For Sun transits, only planets with orbits inside earth's. So that means just Mercury and Venus.

Venus transited the Sun twice in the early 2000s. It's not going to happen again for over 90 years. Mercury transits occur more frequently, although not frequently. The next one is 2032.

Conjunctions

Conjunction

When two objects appear near each other in the sky. See also: Astronomical Conjunction

As I alluded to at the beginning of the stream, I personally find conjunctions more interesting than a number of planets in the sky at the same time. The best conjunctions involve two planets getting very close in the sky.

On August 14, 2024 there be a conjunction between Mars and Jupiter. They will be less than the width of the moon apart. It will be the best conjunction this year.

Occultations

Occultation

When a larger object crosses over a smaller object in the sky See also: Occultation

Because it requires a larger object to cover a smaller object in the sky, the most common occultations are when the moon covers a planet. Technically, a total solar eclipse is an occultation.

There are several lunar occultations of planets a year. On September 17th this year, the moon will cover Saturn, but it will only be visible from the western part of the US.

Sometimes a planet will cover a distant star. The next time a bright star is occulted by a planet is 2044 when Venus covers the star Regulus.

Fun fact: Uranus's rings were discovered when it occulted a star.

Interlopers

Comets. They can be impressive photographically. Mostly ok.

Collisions

Comet Levy-Shoemaker hit Jupiter back in the 1990s. I'm not sure if any earth-based telescopes observed it at the time, but with today's technology, amateurs would have been able to have image the aftermath on Jupiter. (It left marks in Jupiter's atmosphere. See the above article for details.)

What about things that hit the moon? It turns out there is software that can help automate it. The planetary imaging tool FireCapture has a plugin that looks for flashes when something hits the moon. Software from the same team has been used to detect over 160 flashes of meteoroids impacting the moon.

Changing things

Sun spots. With a solar telescope, you can see prominences. The latter change over the timescale of minutes and hours and can be quite addictive to watch. The biggest downside is that it requires specialized equipment.

Variable things

Variable stars

Some are regular, some irregular, some cataclysmic. An example of the latter is T Coronae Borealis.

There are a bunch of different causes of variable stars. That is a topic for another day.

Supernova

Using the List of bright supernova, it's quite easy to take a photo of a supernova in another galaxy. Supernovas can occur in our galaxy, but there haven't been any observed for a few centuries.

Galaxies

"Active galaxies" can vary in brightness. Quasars are a subset of active galaxies, and are some of the most intrinsically bright things in the universe. They're powered by supermassive black holes emitting massive amounts of electromagnetic radiation. Because of the variations in their relativistic jets, they can vary in brightness by a measurable amount.