Gaṇa+Iśa or Gaṇa+Pati.
Pati or Iśa means ruler or lord. The word Gaṇa has various meanings:

• Gaṇa: A company, any assemblage or association of men formed for the attainment of the same aims. Here the leader of this assembly is called Gaṇapati and guides the assembly. If this was for a group of Jyotiṣa the leader would be called Jyotir-Gaṇapati. The Mahārāja of Puri was called Gaṇapati due to his function of directing and appointing the work of the people.
• Gaṇa –devata: troops or 9 classes of Deva considered as Śiva’s attendants and under the special superintendence of Gaṇeśa. These are: Āditya (12), Viśva (13), Vasu (8), Tuṣita (36), Ābhāsvara (64), Anila (49), Mahārājika, Sādhya (17) and Rudra(11).
• Gaṇa: A small body of troops (3 Gulmas or 27 chariots and as many elephants, 81 horses, and 135 footmen).
• Nakṣatra- Gaṇa: A series or group of asterisms or lunar mansions classed under three heads (that of the Deva (divine beings), that of the Manuṣya (men), and that of the Rākshasas (merciless beings).
• Gaṇa-chhandas: a metre a foot or four instants
• Gaṇā: Name of one of the mothers in Skanda’s retinue Ahar-, Marūd-, Vṛṣa-, Sā-, Saptā-, Sārva-, Deva-, Mahā-, and Vida-gaṇā

Vedic significance of Ganesa

Birth of Ganesa

The divine mother Parvati, who is the spouse of Śri Śiva, felt vulnerable when she went to bathe in her personal bath house. To avoid the embarrassment of someone intruding whilst she was bathing, she decided to create a guard for the bath house. In the month of Bhadrapada (Sun in Leo-Virgo) as the Tithi was Śukla Chaturthi (4th lunar day of the bright half) the divine mother molded a figure of a boy in mud and infused this boy with life. Awakened with new life the boy saw his mother and was instructed by her to guard off all intruders whilst she was using the bath house.

The boys loving servitude towards his mother, and mother Parvatī’s newfound privacy was a solution which benefited both. Yet, when Śri Śiva wished to visit his spouse in the bath house, the boy indiscriminately refused his entry. Taken back by the boy’s unhesitant refusal of Śri Śiva, the father of the universe, Śri Śiva angrily sought to infuse some sense into the child. The child new no father of his own and saw Śri Śiva’s advise as nonsense falling on deaf ears.

Śri Śiva left the area and instructed his troop of gaṇas to remove the boy from the premises of the bath house. Each of the gaṇas tried hard to remove him but the boy with the sole use of a club managed to defeat any attempt to remove him from the premises. In the mean time Śani had moved into such a position to view the lagna/head of the child.

Finally the gaṇas sought refuge in Śri Śiva to remove the boy. Śri Śiva then angrily went to the bath house and threw his disc-like weapon, the Chakra, which severed the child’s head from the body. The mother Parvatī then came out of the bath house and saw the horrid sight of her dead child. Her cry sent a shriek through all the inhabitants of the world and in anger she unleashed an army of pisachas and pretas (malign spirits) on the army of Śri Śiva. A war waged and with both Śri Śiva and Śri Parvatī being less than amicable towards each other, the war grew larger until it affected all the denizens of the heavens and earth.
Finally, Śri Viṣṇu and Śri Brahmā intervened and Śri Viṣṇu pleaded with Śri Śiva to restore the child back to life. After much talk, Śri Śiva agreed to restore the child’s life and asked that his gaṇas move in the north-eastern direction (blessing of Iśāna) and find the head of the first dead being that they find. They found an elephant, severed its head and brought it to Śri Śiva. Śri Śiva then placed the elephant head on the body of the child and infused it with life. He then gave the newly born the title of Gaṇapati (Gaṇa+pati) or Gaṇeśa (Gaṇa+iśa) as he due to his undefeated prowess showed himself as worthy to delegate the work of the Gaṇas.

Ṛg Veda

The Rg Veda (2.23.1 & 10.112.9) eulogizes Gaṇeśa as being [1] the first worshiped, [2] the lord of all seers, [3] omniscient, [4] the wisdom of the wise, [5] lord of all souls, [6] the guide to success of all actions.
Just as Gaṇeśa protected the gate to his mother so also he guards the door towards God. He is therefore worshipped as the first one before initiating any penance and also the one granting spiritual progress. So also he symbolically grants success in any field of karma, signified by the ten digpala, and removes all obstacles in this pursuance and is called Vighnesvara.
Just as Gaṇeśa did not know his father, so also he symbolizes the weakness in every being of not knowing our real cause of creation, namely God. His weakness and later immortality granted by his father is worshipped to overcome the ignorance that exists in every being and answer the question: who is my real father?
Just as he was revived with the blessing of a new head and thus new intellect, so also Gaṇeśa is worshipped to instill divine intelligence into us by God, and let go of all past and wrong tendencies which had led us astray in the past. This is symbolic of the highest accomplishment and is worshipped as Siddhi-da or the giver of Siddhis/accomplishments.
The head being of a different species as the body shows the attainment of divine intellect in a body of senses.

Gaṇapati in Jyotiṣa

Our state of being without a head is seen from the graha Ketu in the chart. In the negative it causes problems from the ethereal world by attracting spirits suffering from the anger (Mars) or sorrow (Saturn) experienced in their mortal abode, which in turn affect the body (1st/8th) and health negatively.
If it affects ones career (Mercury) and marriage (Venus) it causes disillusionment and disappointments as well as great difficulty in enjoying the most basic desires for life. Yet, can grant the ability to perform great penance and establish purity when associated with Venus.
In the positive it gives self realization (Sun), selflessness and supreme intuition (Moon) and the blessings of the sampradaya or tradition (Jupiter).
Focusing on the spiritual direction, Maharishi Paraśara and Jaimini explain that the key to open our door towards God lies in the navāmśa placement of Ketu. Further this gives Siddhi and great spiritual depth.

रविकेतुभ्यां शिवे भक्तिः॥७२॥
raviketubhyāṁ śive bhaktiḥ||72||
Translation: If Sun is associated with Ketu then Śiva worship is indicated.
Prasiddha Mantra: namah śivāya|

चन्द्रेण गौर्यां॥७३॥
candreṇa gauryāṁ||73||
Translation: If Moon associates then Gouri worship is indicated.
Prasiddha Mantra: om matre namah|

शुक्रेण लक्ष्म्याम्॥७४॥
śukreṇa lakṣmyām||74||
Translation: If Venus associates then Lakshmī worship is indicated.
Prasiddha mantra: om lakṣmi-nārāyaṇāya namah|

कुजेन स्कन्दे॥७५॥
kujena skande||75||
Translation: If Mars associates then Skanda worship is indicated.
Commentary: Rudra or Murugan worship is advised to avoid the negatives of dwelling on violence or anger.
Prasiddha mantra:om namo bhagavate rudrāya|

बुधशानिभ्यां विष्णौ॥७६॥
budhaśānibhyāṁ viṣṇau||76||
Translation: For both Mercury and Saturn Viṣṇu worship is indicated.
Prasiddha mantra:

1. om viṣṇave namah| for Mercury
2. om namo nārāyaṇāya| for Saturn

गुरुणा साम्बशिवे॥७७॥
guruṇā sāmbaśive||77||
Translation: If Jupiter associates then Sāmbaśiva worship is indicated.
Prasiddha mantra:

1. om bhurbhuvah svah sāmbasadaśivāya namah|
2. om tat sat|

राहुणा तामस्यां दुर्गापांच॥७८॥
rāhuṇā tāmasyāṁ durgāpāṁca||78||
Translation: If Rāhu associates then Tāmasī or Durgā worship is indicated.
Prasiddha mantra: om aiṁ hrīṁ klīṁ camuṇḍayai viccai|

केतुना गणेशे स्कन्दे च॥७९॥
ketunā gaṇeśe skande ca||79||
Translation: If Ketu alone associates then Gaṇeśa or Skanda worship is indicated.
Prasiddha mantra: om gaṇeśāya namah|

Means of interpretation

The interpretation of the placement of Ketu is to look at its navāmśa position.

• If Ketu is joined a planet, then that planet will indicate the Devatā to be worshipped.
• If Ketu is joined more than one planet, then the stronger of those planets, excluding Ketu, will indicate the Devatā. Strength is based on exaltation, mulatrikona, own sign and if inconclusive then the planet with higher degrees is stronger.
• If Ketu is unconjoined then the lord of the sign where he is placed indicates the Devatā. If this happens in Scorpio and Mars is weakly placed, then the Devatā is Gaṇeśa himself.
• If a planet has raśi dristi on Ketu’s navāmśa position and happens to be Iṣṭa-, Dharma-, Palaṇa- or Guru Devatā, then the worship of the indicated Devatā can lead to great and deep spiritual accomplishments.

Chart 1: Example

In the chart of Śri Ramakrishna Pāramahamsa, Ketu is placed in the navāmśa of Cancer and is joined Mars. Mars indicates that the worship of Rudra or Skanda is advised. Yet, Mars and Moon are in parivartana yoga between the signs of Aries and Cancer and indicates that instead worship of Śri Gourī is indicated. Moon is the Bhratrikāraka and shows that the Gurudevatā is also Śri Gourī and will bring the experience of pratibha or direct communion with the Guru.

The Ishta Devatā (twelfth from karakāmśa) in the chart is seen from Sagittarius housing the Sun indicating Śiva, Rama, Matangi or a similar Devatā indicated by the Sun. As this also the Palana Devatā (sixth from amatyakaraka), the Ista Devatā is more likely to take on the form of the mother Devī.
However, these neither aspect nor associate otherwise with Ketu. Instead the lord Jupiter (Tarā) is joined Saturn (Kalī) and Mercury (Tripurā or Radhā). Note that Mercury is also the ninth lord from karakāmśa and will indicate the Dharmadevatā and thus indicate a Viṣṇurūpa, therefore among Tripura and Radhā, the latter would be the natural choice.

All three planets share a mutual raśi dristi with Cancer housing Ketu, showing that Siddhi and deep spiritual experiences can arise from their worship. Śri Rāmakrishna used to worship Śri Kalī (Saturn) and Śri Radhā (Moon) at the Bhavatarinī (Jupiter) temple. Notice that all the Ishta Devatā are the forms of the mother because they also indicate the Palaṇa Devatā which is always the form of a Devī and therefore literally are his Ishta Devī and the supporters of his spiritual experiences (Ketu).

Chart 2: Example 2

In the navāmśa of Śri Achyuta Dasa, Ketu is placed in Libra navāmśa. Ketu is unconjoined as its lord is Venus (Lakṣmī rūpa being defined). Venus is placed in Cancer showing that this is Lakṣmī in the form of the great giver of food, i.e. Annapurëeśvarī. Venus is also the Dharmadevatā (joined ninth lord from karakāmśa) and therefore becomes a Viṣṇu-avatara. Venus is the dispositor of both Ketu and Jupiter and therefore causes Sampradāya yoga and brings a tradition of Jyotish (Ketu). Jupiter is the Bhratrikaraka and therefore indicates that this will be instigated by the dikṣa guru, who in this case was Śri Chaitanya Mahāprabhu whom also gave Achyutānanda his Viṣṇu mantra and re-ignited the Jyotish-Parampara of Orissa.

The Ishta Devatā is seen from Scorpio and Rāhu and Moon are placed in it. Rāhu and Moon are debilitated and indicate a form of the divine mother. Neither are joined nor aspecting Ketu, but being in the sign of Ketu (Scorpio) they indicate Siddhi and divine knowledge through the worship of the mother. Śri Achyutānanda Dasa was a regular at the Gouri-Mangalā temple (Moon in Scorpio) in Kagatpur, Orissa.