- May 18, 2016 - Planétarium : Astronomical News
For the past month, night owls and early risers have doubtlessly noticed a dazzling, orange star-like object shining in the south. That object is the notorious Red Planet, Mars… and it’s been getting brighter, and appearing earlier with each passing night.
Mars is usually inconspicuous, but for several weeks every 26 months, as it approaches opposition, it distinguishes itself against the starry background by growing larger and brighter. So what, exactly, is opposition, and why does it make Mars brighter?
Closing the gap
Oppositions occur whenever the Earth and an outer planet align on the same side of the Sun. As such, the term “opposition” is reserved for planets whose orbits are larger than ours. But to understand why Mars gets brighter during opposition, it helps to visualise the orbital paths of the planets in question.
The Earth circles the Sun at a velocity of 30 km/s and takes 365 days to complete an orbit; Mars, which has a larger orbit, travels at 24 km/s and its “year” takes 687 days. As both planets race around the Sun, in the time it takes Earth to complete its annual circuit, Mars continues to advance: The result is that Earth eventually manages to overtake the Red Planet, but this takes precisely 780 days. When Earth and Mars are again neck and neck, Mars is said to be in opposition—on the side of Earth exactly opposite the Sun—and around that time, the gap between the two planets closes to a minimum.
The closer, the brighter
Since the Earth and Mars have elliptical orbits the distance between them during opposition can vary. When Earth is at perihelion (its closest point to the Sun) and Mars is at aphelion (its farthest point from the Sun), the gap between them reaches a maximum of about 101 million km. But when the tables are turned during opposition, and Earth is at aphelion while Mars is at perihelion, the Earth-Mars distance narrows to about 56 million km: This is known as a perihelic opposition.
As a result, Mars’ apparent size at opposition can grow from about 13.8 to 25.7 arc seconds—nearly two times larger! Likewise, its brightness can increase from apparent magnitude –1.2 to –2.9 —nearly 5 times brighter!
This time around
When the Red Planet reaches opposition on May 22, it will rise at about 8:20 p.m. EDT, just around sunset, culminate at 0:45 a.m., 23° above the southern horizon, and set at 5:13 a.m. as the Sun rises. However, due to eccentricities in the orbits of Earth and Mars, the Red Planet’s closest approach normally varies by a few days on either side of opposition. This time around, Mars will be closest to us (75.3 million km), largest (18.6 arc seconds) and brightest (magnitude –2.06) on the evening of May 30.
