Ketu (केतु) is the Descending (South) Lunar Node in Vedic (वैदिक) or Hindu Astrology (हिंदू ज्योतिष).
Ketu (केतु) is generally referred to as a "Shadow" Planet.
It is believed to have a tremendous impact on human lives and also the whole creation.
In some special circumstances it helps someone achieve the zenith of fame.
Ketu (केतु) is often depicted with a gem or star on his head signifying a mystery light.
- Jyotisha or Jyotishya (ज्योतिष, ज्योतिश्या / "light, heavenly body") is the traditional Hindu system of Astrology, also known as Hindu Astrology (हिंदू ज्योतिष) or Vedic Astrology (वैदिक ज्योतिष).

Ketu (केतु) is the Lord of three Nakshatras or Lunar Mansions (नक्षत्र): Ashvini (अश्विनी), Magha (मघा) and Mula (मोला, Moola).
- The Nakshatras or Lunar Mansions (नक्षत्र) are 27 equal divisions of the night sky used in Hindu Astrology (हिंदू ज्योतिष / Jyotisha, ज्योतिष / Jyotishya, ज्योतिश्या / Vedic Astrology, वैदिक ज्योतिष), each identified by its prominent star(s).
Historical (Medieval) Hindu Astrology (हिंदू ज्योतिष) enumerated either 27 or 28 Nakṣatras (नक्षत्र).
In modern Astrology, a rigid system of 27 Nakṣatras (नक्षत्र) is generally used, each covering 13° 20′ of the Ecliptic.
The missing 28th Nakshatra (नक्षत्र) is Abhijeeta (अभिजीत).
Each Nakṣatra (नक्षत्र) is divided into equal quarters or Padas (क्वार्टर) of 3° 20′.
Of greatest importance is the Abhiśeka Nakṣatra (अभिसेका नक्षत्र), which is held as king over the other Nakṣatras (नक्षत्र).
Worshipping and gaining favour over this Nakṣatra (नक्षत्र) is said to give power to remedy all the other Nakṣatras (नक्षत्र), and is of concern in predictive Astrology and mitigating Karma (कर्म).
- The Ecliptic is the mean plane of the apparent path in the Earth's sky (पृथ्वी) that the Sun (सूर्य, Surya) follows over the course of one year.
- Karma (कर्म) means action, work or deed; originating from India.
It also refers to the spiritual principle of cause and effect where intent and actions of an individual (cause) influence the future of that individual (effect).
Good intent and good deeds contribute to good Karma (कर्म) and happier Rebirths (पुनर्जन्म / Punarjanma), while bad intent and bad deeds contribute to bad Karma (कर्म) and bad Rebirths (पुनर्जन्म / Punarjanma).
The philosophy of Karma (कर्म) is closely associated with the idea of Rebirth (पुनर्जन्म / Punarjanma) in many schools of Indian religions, particularly Hinduism (हिंदू धर्म), Buddhism (बौद्ध धर्म), Jainism (जैन धर्म) and Sikhism (सिख धर्म), as well as Taoism (道教).
In these schools, Karma (कर्म) in the present affects one's future in the current life, as well as the nature and quality of future lives - one's Saṃsāra (संसार).

Ketu (केतु) is considered responsible for Moksha (मोक्ष), Sannyasa (संन्यास), self-realization, Gnana (ज्ञान, Jñāna), a wavering nature, restlessness, the Endocrine System and slender physique.
- Moksha (मोक्ष, mokṣa / विमोक्सा, vimoksha / विमुकटी, vimukti / मुक्ति, mukti) is a term in Hinduism (हिंदू धर्म), Buddhism (बौद्ध धर्म), Jainism (जैन धर्म) and Sikhism (सिख धर्म), which refers to various forms of Emancipation, Enlightenment, Liberation, and Release.
In its soteriological and eschatological senses, it refers to freedom from Saṃsāra (संसार), the Cycle of Death and Rebirth (पुनर्जन्म / Punarjanma).
In its epistemological and psychological senses, Moksha (मोक्ष) refers to freedom from ignorance: self-realization, self-actualization and self-knowledge.
In Hindu traditions, Moksha (मोक्ष) is a central concept and the utmost aim to be attained through Three Paths during human life.
These Three Paths are: Dharma (धर्म / "virtuous, proper, moral life"), Artha (अर्थ / "material prosperity, income security, means of life"), and Kama (काम / "pleasure, sensuality, emotional fulfillment").
Together, these four concepts are called Puruṣārtha (पुरुषार्थ) in Hinduism (हिंदू धर्म).
- Sannyasa (संन्यास, Saṃnyāsa) is the life stage of Renunciation within the Hindu Philosophy of four age-based life stages known as Ashramas (आश्रम), with the first three being: Brahmacharya (ब्रह्मचर्य / "bachelor student"), Grihastha (गृहस्थ / "householder") and Vanaprastha (वनप्रस्थ / "forest dweller, retired").
Sannyasa (संन्यास) is traditionally conceptualized for men or women in late years of their life, but young Brahmacharis (वनप्रस्थ, Brahmacharya / "one who gives up worldly life") have had the choice to skip the householder and retirement stages, renounce worldly and materialistic pursuits and dedicate their lives to spiritual pursuits.
Sannyasa (संन्यास) is a form of asceticism, is marked by renunciation of material desires and prejudices, represented by a state of disinterest and detachment from material life, and has the purpose of spending one's life in peaceful, love-inspired, simple spiritual life.
An individual in Sanyasa (संन्यास) is known as a Sannyasi (संन्यासी / male) or Sannyasini (संन्यासिनी / female) in Hinduism (हिंदू धर्म), which in many ways parallel to the Sadhu (साधु) and Sadhvi (साध्वी) traditions of Jain Monasticism (जैन मुनि), the Bhikkhus (भिक्षु, Bhikṣu) and Bhikkhunis (भिक्षुणी, Bhikṣuṇī) of Buddhism (बौद्ध धर्म) and the Monk and Nun traditions of Christianity, respectively.
- In Indian Philosophy and Religion, Jñāna (ज्ञान, Gnana or ग्नान, Gñān, Gyan/Gian) is "knowledge".
The idea of Jnana centers on a cognitive event which is recognized when experienced.
It is knowledge inseparable from the total experience of reality, especially a total or Divine Reality (ब्रह्मन्, Brahman).
- Soteriology is the study of Religious Doctrines of Salvation.
Salvation theory occupies a place of special significance in many Religions.
- Eschatology is a part of Theology concerned with the Final Events of History, or the Ultimate Destiny of Humanity.
This concept is commonly referred to as the "End of the World" or "End Times".
- Epistemology is the branch of Philosophy concerned with the Theory of Knowledge.
- Puruṣārtha (पुरुषार्थ / "object of human pursuit") is a key concept in Hinduism (हिंदू धर्म), and refers to the four proper goals or aims of a human life.
The four Puruṣārthas (पुरुषार्थ) are: Dharma (धर्म / Righteousness, Moral Values), Artha (अर्थ / Prosperity, Economic Values), Kama (काम / Pleasure, Love, Psychological Values) and Moksha (मोक्ष / Liberation, Spiritual Values).

In Hindu Astrology (हिंदू ज्योतिष), Ketu (केतु) represents Karmic collections both good and bad, spirituality and supernatural influences.
Ketu (केतु) signifies the spiritual process of the refinement of materialization to spirit and is considered both malefic and benefic, as it causes sorrow and loss, and yet at the same time turns the individual to God.
In other words, it causes material loss in order to force a more spiritual outlook in the person.

Ketu (केतु) is a Karaka (संकेतक, Sanketak) or indicator of intelligence, wisdom, non-attachment, fantasy, penetrating insight, derangement, and psychic abilities.

Ketu (केतु) is believed to bring prosperity to the devotee's family, removes the effects of snakebite and illness arising out of poisons.
He grants good health, wealth and cattle to his devotees.

The people who come under the influence of Ketu (केतु) can achieve great heights, most of them spiritual.

Ketu (केतु), being a karmic planet would show the necessity and urge to work on a specific area of life where there had been ignorance in the past life.
To balance the apparent dissatisfaction one has to go that extra mile to provide a satisfactory settlement in the present lifetime.

According to the most popular astrology text, Brihat Parashara Hora Shastra, (बृहत पराशर होरा शास्त्र) Ketu (केतु) is related to Matsya (मत्स्य).
- Matsya (मत्स्य) is the Fish Avatar (अवतार) in the Ten Primary Avatars (दशावतार, Daśāvatāra) of Hindu God, Vishnu (विष्णु).
Matsya is described to have rescued Manu (मनु) and earthly existence from a great deluge and one of the first sapient beings to evolve on the Earth (पृथ्वी, Prithvi).
- Avatar (अवतार, Avatāra / "descent") is the Material Appearance or Incarnation (अवतारवाद) of a Deity on Earth (पृथ्वी, Prithvi).
- Vishnu (विष्णु) is one of the Principal Deities of Hinduism (हिंदू धर्म), and the Supreme Being or Absolute Truth in its Vaishnavism Tradition (वैष्णववाद).
Vishnu (विष्णु) is the "preserver" in the Hindu triad, Trimurti (त्रिमूर्ति) that includes Brahma (ब्रह्मा) and Shiva (शिव).
- Brahma (ब्रह्मा) is the Creator God in Hinduism (हिंदू धर्म).
He is also known as Svayambhu (स्वयम्भू / "self-born") or the creative aspect of Vishnu (विष्णु), Vāgīśa (वगिसा / "Lord of Speech"), and the creator of the four Vedas (वेद), one from each of his mouths.
- Shiva (शिव / "the auspicious one") also known as Mahadeva (महादेवा / "the great God") is one of the Principal Deities of Hinduism (हिंदू धर्म).
He is one of the supreme beings within Shaivism (शैव संप्रदाय), one of the major traditions within contemporary Hinduism (हिंदू धर्म).
- Manu (मनु) is a term found with various meanings in Hinduism (हिंदू धर्म).
In early texts, it refers to the archetypal man, or to the first man (Progenitor of Humanity).
The Sanskrit term (संस्कृत) for "human", (मानव, Mānava) means "of Manu" or "children of Manu".

Ketu (केतु) is usually paired with Rahu (राहु).
Astronomically, Ketu (केतु) and Rahu (राहु) denote the points of intersection of the paths of the Sun (सूर्य, Surya) and Moon (चन्द्र,Chandra) as they move on the Celestial Sphere.
Therefore, Ketu (केतु) and Rahu (राहु) are respectively called the South and the North Lunar Nodes.
The fact that Eclipses (चंद्रगहण) occur when the Sun (सूर्य, Surya) and Moon (चन्द्र,Chandra) are at one of these points gives rise to the understanding of swallowing of the Sun (सूर्य, Surya) and Moon (चन्द्र,Chandra) by the snake.
- A Lunar Node is either of the two Orbital Nodes of the Moon (चन्द्र,Chandra), that is, the two points at which the Orbit of the Moon (चन्द्र,Chandra) intersects the Ecliptic.
The Descending (or South) Node is where the Moon (चन्द्र,Chandra) enters the Southern Ecliptic Hemisphere, while the Ascending (or North) Node is where the Moon (चन्द्र,Chandra) moves into the Northern Ecliptic Hemisphere.
- The Ecliptic is the mean plane of the apparent path in the Earth's sky (पृथ्वी) that the Sun (सूर्य, Surya) follows over the course of one year.

A Lunar Eclipse (चंद्रगहण, Chandragahan) can occur only when the Full Moon (पूर्णिमा, Purnima) is near (within 11° 38' ecliptic longitude) either Lunar Node, while a Solar Eclipse (सूर्यग्रहण, Suryagrahan) can occur only when the New Moon (अमावस्या, Amāvásyā) is near (within 17° 25') either Lunar Node.

As per Vedic Astrology, Ketu (केतु) and Rahu (राहु) have an orbital cycle of 18 years and are always 180 degrees from each other orbitally (as well as in the Birth Charts).
This coincides with the precessional orbit of Moon (चन्द्र,Chandra) or the ~18 year rotational cycle of the lunar ascending and descending nodes on the Earth's ecliptic plane (पृथ्वी).
This also corresponds to a Saros, a period of approximately 223 synodic months (approximately 6585.3211 days, or 18 years, 11 days, 8 hours), that can be used to predict eclipses of the Sun (सूर्य, Surya) and Moon (चन्द्र,Chandra).

Ketu (केतु) rules the zodiac sign of Scorpio (in Hindu Astrology: वृश्चिक, Vṛścika) together with Mangala (मङ्गल).
- Mangala (मङ्गल) is the name for Mars, the red planet, in Hindu texts.
Also known as Lohit (लोहित / "Made of Iron"), he is born from Shiva's (शिव) sweat or blood drop.
He is also a god of war, celibate and sometimes linked to God Karttikeya (कार्तिकीया, "of the Krittikas" / मुरुगन, Murugan / स्कंद, Skanda, "Leaper or Attacker"/ कुमार, Kumara / सुब्रह्मण्य, Subrahmanya / स्वामीनाथ, Swaminatha, "Ruler of the Gods" / सरवनभव, Śaravaṇabhava, "Born amongst the Reeds" / अरुमुगम or सांमुखा, Arumugam or Ṣaṇmukha, "Six-Faced" / डंडापानी, Dandapani, "Wielder of the Mace"), the Hindu God of War.
His origins vary with different Mythological texts; in some, he is the son of Bhumi (भूमि), the Avatar of Goddess Lakshmi (लक्ष्मी / representing Mother Earth) and Vishnu (विष्णु), born when he raised her from the depths of water during the time of the Varaha avatar (वराह).
- Lakshmi (लक्ष्मी) is the Hindu Goddess of wealth, fortune and prosperity.
- Varaha (वराह / "boar") is the avatar of the Hindu God, Vishnu (विष्णु), who takes the form of a boar to rescue Goddess Earth (भूमि, Bhumi).
Varaha is listed as Third in the Dashavatara (दशावतार), the Ten Principal Avatars of Vishnu (विष्णु).

Often Ketu (केतु) is misunderstood as Uranus (यूरेनस) during Sanskrit (संस्कृत) to English translation.