2025, January 25-February 2:  Sky Watching – Venus is Guide to Neptune

January 25-February 2, 2025:  Brilliant Evening Star Venus is a guide to Neptune, the most-distant planet in the modern solar system model.

2022, February 27: Venus, Mars, and the crescent moon in the southeast before sunrise.
Photo Caption – 2022, February 27: Venus, Mars, and the crescent moon in the southeast before sunrise.

by Jeffrey L. Hunt

The planets’ locations are described relative to sunset.  Find the time in local sources.

Venus is Guide to Neptune

Brilliant Venus is a guide to Neptune, the solar system’s farthest planet from the sun, after Pluto was demoted to dwarf planet status.

Spot Four Bright Planets Nightly
Chart Caption – 2025, January 25-February 2: Venus approaches and passes Neptune through a binocular.

Neptune is quite dim and it is in front of an unremarkable Pisces starfield, unlike Uranus that is near the Pleiades and a richer starry background.

Venus is moving near Neptune, passing closest on the 31st, with a gap of 3.2°.  While the two appear close together against the distant stars, they are nearly 3 billion miles apart in space!

Venus revolves around the sun in nearly 225 days, while Neptune’s year is almost 165 years.

Neptune’s Discovery

This contrast enhanced color picture of Neptune was acquired by Voyager 2 at a range of 14.8 million kilometers (9.2 million miles) on August 14, 1989. (NASA photo)
Photo Caption – This contrast enhanced color picture of Neptune was acquired by Voyager 2 at a range of 14.8 million kilometers (9.2 million miles) on August 14, 1989. (NASA photo)

Neptune’s presence was predicted by its gravitational influence on Uranus that was not moving through its solar orbit as expected.  The planet’s first observation was made at the Berlin Observatory by Johann Galle and Heinrich d’Arrest, after the pair pleaded with the director Johann Encke.  Galle received predictions of the planet’s location from Jean Joseph (J.J.) Leverrier, a French astronomer who had labored over complex mathematical equations to predict the unknown planet’s celestial location.  The two observers found it quite easily on the first night of observing during the night of September 23/24, 1846.

Neptune, seven of its 18 satellites, rings, and atmospheric features are seen in the Webb Space Telescope infrared view of the planet
Photo Caption – Neptune, seven of its 18 satellites, rings, and atmospheric features are seen in the Webb Space Telescope infrared view of the planet. Triton is very bright in the image because its surface is highly reflective while Neptune’s atmosphere absorbs infrared making it appear dimmer (NASA Photo).

John Couch Adams, an English mathematician, made a similar prediction, but he was unable to convince the Royal Observatory’s director, known as the Astronomer Royal, George Airy, to look for the planet.  Adam’s letter with the prediction languished for eight months from Airy’s inaction.

The story of the first sighting is described in the book The Discovery of Neptune by Morton Grosser.  It can be found in used online bookstores. The 140-page story is mostly without technical jargon and mathematics.

Neptune’s location at discovery is a little west of Saturn’s current location in the sky.  Since that first sighting, the planet has orbited once and moved to its current location.

Find Neptune after Sunset

Venus is Guide to Neptune
Chart Caption – 2025, January 29: An hour after sundown, Venus and Saturn are in the southwest. Use a binocular to look for Neptune near Venus.

The accompanying chart shows Venus, Saturn and Neptune and hour after sunset on January 29th.  From January 25th through February 2nd, Venus and Neptune are in the same binocular field.  The planet is quite dim and darkness is needed to see it. At the mid-northern latitudes, twilight ends about 90 minutes after sunset.  For the next 30 to 45 minutes the planet is still high enough in the sky to avoid most of the planet’s blurring and filtering affects near the horizon.

In addition, the moon passes by during early February.  From February 3rd through 13th, moonlight washes across the sky veiling dimmer objects and making a Neptune sighting even more difficult.  By Valentine’s Day night, Neptune is less than 15° above the horizon at the Beginning of the viewing window and subject to atmospheric extinction.

Be Persistent

To see the planet, this sighting may take more than one night.  Here’s how to see Neptune.  At 90 minutes after sunset, locate brilliant Venus, nearly 30° up in the west.  While not visible to the unaided eye in any location, even the darkest locations on the planet, Neptune is to the left of the Evening Star.

Venus is Guide to Neptune
Chart Caption – 2025, February 1: Through a binocular, the crescent moon appears with Venus and Neptune.

Place Venus in the binocular field.  Depending on the night, hold the binocular so that Venus, 20 Piscium (20 Psc on the chart) and 24 Piscium (24 Psc) are in the field of view.  For scale, the distance between the stars is 1.3°.  Dimmer than the reference stars and appearing as a bluish star, Neptune is nearly 1° to the upper right of 24 Psc and 1.3° to 20 Psc’s upper left. As noted above, the Venus-Neptune conjunction (3.2°) occurs on the 31st.

Look for this distant world, using Venus as the guide.

LATEST ARTICLES